Affecting 1 in 68 children, Autism Spectrum Disorder (ASD) is one of the most prevalent cognitive disorders in the global population. Symptoms of ASD, although typically not life-threatening, have a large impact on the social wellbeing of diagnosed individuals. Inflammation in the brain, or neuroinflammation, has previously been shown to increase the severity of the behavioral deficits associated with ASD. The exact etiology of the neuroinflammation observed in ASD remains unclear, especially in regards to protein expression that initiates the inflammatory pathway. This experiment examines two specific markers of neuroinflammation, glial fibrillary acidic protein (GFAP) and myelin-associated glycoprotein (MAG) in a previously characterized mouse model of ASD. GFAP is astrocyte-specific, cytoplasmic, and has been shown to be upregulated in trauma or disease pathologies in the brain. MAG is found in the membrane of oligodendrocytes and is a major regulator of development and regeneration of nervous tissue. Control C57bl/6j mice and ASD-representative BTBR T+tf/J (BTBR) mice were sacrificed twenty-one days after birth. Immunoblotting was performed on cingulate cortical tissue using anti-GFAP and anti-MAG primary antibodies to quantify levels of GFAP and MAG protein expression between the control and ASD models. These findings provide further evidence that changes in GFAP and MAG expression may alter the neuroinflammatory pathways observed in ASD-representative mice.

Trafficking of protein and lipid cargo through the secretory pathway in eukaryotic cells is mediated by membrane-bound vesicles. Secretory vesicles are targeted to sites of exocytosis on the plasma membrane in part by a conserved multi-subunit protein complex termed the exocyst. In addition to tethering vesicles to the plasma membrane, the exocyst complex and components therein may also add a layer of regulation by directly controlling assembly of the SNARE complex, which is required for membrane fusion, as well as other regulatory factors such as Sec1p. In the past, we have shown that Sec6p interacts with Sec9p in vivo and that that interaction retards binary SNARE complex formation in a SNARE assembly assay. Though many interactions have been mapped using in vitro methods, confirming them in vivoand placing them into the context of a complete model that accounts for all observed interactions (and lack of interactions) has proven difficult. In order to address these problems, I have studied the interactions between Sec6p and other factors involved in exocytosis at the plasma membrane via in vivo methods. My hypothesis was that Sec6p interaction with Sec9p and subsequent inhibition of SNARE complex assembly in vitro was an intermediate state and Sec6p was part of a set of cofactors that accelerated SNARE complex assembly in vivo. To test this hypothesis I showed that the interaction between the plasma membrane t-SNARE Sec9p and the yeast exocyst subunit Sec6p can be observed in vivoand designed point mutations to disrupt that interaction. Interestingly, I also showed that Sec6p:Sec9p interaction involves the free pool of Sec6p rather than the exocyst bound fraction of Sec6p. Point mutations in the N-terminal domain of Sec6p result in temperature sensitive growth and secretion defects, without loss of Sec6p-Sec9p interaction. However, at the non-permissive temperature, the exocyst subunits Sec5p, Sec10p and Sec15p are mislocalized and are absent from the exocyst complex. The resulting subcomplex, containing Sec3p, Sec8p, Exo70p and Exo84p, remains stably assembled and localized at sites of polarized secretion. This subcomplex is likely due to disruption of interaction between Sec6p and Sec5p, and may be similar to that observed at restrictive temperatures in the sec6-54temperature sensitive mutant. Additionally, one of the sec6 temperature sensitive mutants displays a loss of binding to the yeast regulatory protein Sec1p. In vitro binding studies indicate a direct interaction between Sec1p and the free pool of the wild-type Sec6p protein, suggesting close interplay between Sec6p and Sec1p in the regulation of SNARE complexes. A coherent model which incorporates all these interactions has continued to be elusive. However, the results I have found do suggest several hypotheses which should prove testable in the future.

Axon pathfinding is an important process required for the establishment of proper neuronal connections during development. An increasing number of secreted and membrane-anchored molecules have been identified as axon guidance cues, which can act as positive or negative factors to increase or decrease the growth of axons and influence the direction of axonal growth. These axon guidance factors present in the extracellular environment interact with receptors present on the growth cone, a structure located at the tip of the axon which functions as the motor unit for the axon. Upon binding to their receptors on the growth cone, the guidance factors then elicit an intracellular signaling cascade within the axon that ultimately influences the direction of axon growth, often through a direct, non-transcriptional mechanism. In this dissertation, we show that Sonic hedgehog (Shh) acts as an axon guidance factor for chick retinal ganglion cell (RGC) axons in a concentration-dependent manner. At a low concentration, Shh functions as a positive factor that induces axon growth and attractive turning while, at a high concentration, Shh functions as a negative factor that induces axon retraction and repulsive axon turning. We further characterized the effects of Shh on macropinocytosis, a fluid-phase type of endocytosis, in the axons. A high concentration of Shh significantly increased macropinocytosis in the axons. Macropinocytosis resulted in the generation of large, dextran-positive, clathrinindependent vesicles in the axonal growth cones, prior to growth cone collapse, axon retraction and repulsive axon turning. These vesicles were found to require dynamic F-actin, nonmuscle myosin II and dynamin for their formation but were formed independently of PI3 kinase signaling. Interestingly, a low concentration of Shh had an opposite effect on macropinocytosis. A low concentration of Shh and soluble laminin decreased macropinocytosis and additionally increased the turnover of these vesicles within the axons, suggesting positive axon guidance factors can additionally regulate downstream processing or maturation of these vesicles. The effect of Shh on regulating the motility of macropinosomes within the axons was investigated. A low concentration of Shh appeared to increase the motility of these vesicles along axonal microtubules in a cAMPdependent manner. However, a high concentration of Shh did not appear to affect the motility of the macropinosomes, suggesting that it likely plays a more predominant role in the formation of these vesicles within the growth cone. When we began this work, a large body of research existed describing the effects of guidance factors on regulating the cytoskeleton during axon motility. However, the role of membrane trafficking events during axon growth and guidance were very poorly characterized. Since we began this project, an increasing number of reports have shown that endo- and exocytosis are important for axon growth and, here, we show that macropinocytosis induced by negative axon guidance factors plays a critical role in growth cone collapse, axon retraction and repulsive axon turning. Positive axon guidance factors also affect macropinocytosis within the axons and additionally regulate their maturation, suggesting that membrane trafficking events mediated by axon guidance factors are important for regulating axon growth and pathfinding.

The overall objective of this study was to understand the mechanisms that control antigen processing and binding of peptides to major histocompatibility complex (MHC) molecules. Towards this goal I investigated (a) the structural features of T cell-presented peptides with a focus on the role of recurrent hydrophobic residues in catalysis of helix formation by these peptides and (b) the biochemical events that determine the fates of the invariant chain molecule (Ii) in its various post-translational processing pathways. In the structural studies, I tested the hypothesis that the recurrence of hydrophobic amino acids in a polypeptide at positions falling in an axial, hydrophobic strip if the sequence were coiled as an α-helix can lead to helical nucleation on a hydrophobic surface.For a series of HPLC-purified peptides, including some T cell-presented peptides varying considerably in primary sequence, percentage helicity in the presence of lipid vesicles correlated with strip-of-helix hydrophobicity index (SOHHI), as shown by circular dichroism (CD) analysis. A prototypic helix peptide PH-1.0 (LYQELQKLTQTLK) was designed with a strong axial hydrophobic strip of 4 leucine residues. PH-1.0 formed about 38% helical structure in 10 mM phosphate buffer at pH 7.0 with di-O-hexadecyl phosphatidylcholine (DHPC) lipid vesicles, but no helical structure was detected when the peptide was in phosphate buffer alone. The helix-forming tendencies of 9 analogs of PH-1.0 with one or two amino acid variations from the parent peptide were examined in the presence of lipid vesicles and the results showed that (a) decreasing the strip-of-helix hydrophobicity by substituting even one of the four leucine residues in the axial hydrophobic strip with a less hydrophobic threonine residue reduced the helix-forming tendency of a peptide in the presence of lipid vesicles; (b) the placement of recurrent hydrophobic residues within the axial hydrophobic strip appeared to be critical for a peptide to be induced to form an α-helix by a hydrophobic surface; (c) there was an orientation preference for these peptides to interact with lipid vesicles and to form helical structure; (d) extra hydrophobic residues in other positions appeared to compete with the hydrophobic residues within the axial hydrophobic strip for interaction with the lipid vesicles and therefore to decrease the helix-forming tendency of peptides. For the biochemical studies of the function of Ii, a 17-residue peptide, Ii-3 (Ii 148-164), was synthesized. The CD analysis of Ii-3 showed mainly an α-helical conformation when the peptide was examined in the presence of lipid vesicles. [125I]-labeled Ii-3, after coupling at the N-terminus with a photoactivatable, heterobifunctional crosslinker N-hydroxysuccinimidyl-4-azidobenzoate (HSAB), was able to bind to both α and β chains of class II MHC molecules, indicating that this region of Ii might cover the desetope of class II MHC molecules from the time of their synthesis until their charging with foreign peptides at an endosomal compartment. The biosynthesis of a chondroitin sulfate proteoglycan-form of Ii (CS-Ii) was examined in a class II MHC-negative cell line P3HR-1. [35S]sulfate-labeled microsomal membrane proteins of P3HR-1 were immunoprecipitated with anti-Ii monoclonal antibody and the results of SDS-PAGE analysis demonstrated that P3HR-1 could process Ii to CS-li in the absence of class II MHC molecules and the chondroitin sulfate identity of this molecule was confirmed by chondroitinase-ABC treatment. We conclude that there might be a class II MHC-independent pathway to process Ii to a chondroitin sulfate proteoglycan form as compared to the pathway in which Ii was associated with class II MHC and later cleaved by proteases residing in the endosomal compartment. In an effort to demonstrate in vitro that the class II MHC-associated Ii was eventually dissociated from class II MHC molecules by a proteolytic cleavage process, it was found that cathepsin B could completely remove Ii without damage to class II α and β chains. In order to identify those cleaved Ii fragments, three polyclonal anti-Ii peptide sera were produced by immunizing rabbits with keyhole limpet hemocyanin (KLH)-conjugated Ii peptides. Anti-Ii (146-169) was shown to be able to precipitate a p18 molecule only in cells expressing Ii. Anti-Ii (148-164 )and anti-Ii(78-92) were specific for their respective antigenic peptides as tested by enzyme-linked immunosorbent assay (ELISA).

Cell migration involves complex mechanical interactions between cells or between cells and the underlying substrate. Using a newly developed technique, "traction force microscopy", I have been able to visualize the dynamic characteristics of mechanical forces exerted by migrating fibroblasts such as magnitude, direction, and shear. For NIH 3T3 fibroblasts, I found that the lamellipodium provides nearly all of the force necessary for cell migration. A high shear zone separates the lamellipodium from the remainder of the cell body, suggesting that they are mechanically distinct entities. The timing of the tractions at the leading edge, as well as the spatial distribution, bears no apparent relationship to concurrent local protrusive activities, yet changes in traction force patterns often precede changes in migration direction. In H-ras transformed cells I found isolated regions of weak, transient traction forces in pseudopods all along the cell that appeared to act against one another. The resulting shear pattern suggested that there were multiple disorganized mechanical domains. These results support a frontal towing model for cell migration where the dynamic traction forces at the leading edge served to actively pull the cell body forward. In H-ras transformed cells, the weak poorly coordinated traction forces coupled with weak cell substrate-adhesions were likely responsible for the abnormal motile behavior of these cells. To probe the mechanical interactions beneath various regions of migrating fibroblasts, a cell substrate inhibitor (GRGDTP peptide) was locally applied while imaging stress distribution on the substrate utilizing traction force microscopy. I found that both spontaneous and GRGDTP induced detachment of the trailing edge resulted in extensive cell shortening with no change in overall traction force magnitude or cell migration. Conversely, leading edge disruption resulted in a dramatic global loss of traction forces pnor to any significant cell shortening. These results suggested that fibroblasts transmit their contractile forces to the substrate through two distinct types of adhesions. Leading edge adhesions were unique in their ability to transmit active propulsive forces whereas trailing end adhesions created passive resistance during cell migration and readily redistributed their loads upon detachment. I have also investigated how fibroblasts regulate traction forces based on mechanical input. My results showed that stretching forces applied through the flexible substrate induced increases in both intracellular calcium concentration and traction forces in fibroblasts. Treatment with gadolinium, a well known stretch-activated ion channel inhibitor, was found to inhibit both traction forces and cell migration without inhibiting cellular spread morphology or protrusive activities. Gadolinium treatment also caused a pronounced decrease in vinculin and phosphotyrosine concentrations from focal adhesions. Local application of gadolinium to the trailing region had no detectable effect on overall traction forces or cell migration, whereas local application to the leading edge caused a global inhibition of traction forces and an inhibition of migration. These observations suggest that stretch activated entry of calcium ions in the frontal region serves to regulate the organization of focal adhesions and the output of mechanical forces. Together my experiments elucidate how fibroblasts exert mechanical forces to propel their movements, and how fibroblasts utilize mechanical input to regulate their movements.

DNA mismatch repair is one process in the preservation of genomic integrity. It has been found in Archeae, bacteria, plants, yeast and mammals. The mismatch repair system is highly conserved among species and allows the strand-specific elimination of base-base mispairs, chemical base modifications, as well as short insertion/deletion loops following DNA replication. The repair system also has important effects on homeologous recombination, contributing to the frequency of reciprocal exchanges. In humans, defects in the repair system have been found to be associated with tumorigenesis. In Escherichia coli, this pathway was originally called long patch repair before being renamed the methyl-directed mismatch repair system. It is unique in that it utilizes a DNA methylation pattern to discriminate between the parental DNA strand and the newly synthesized daughter DNA strand. The current model for the initiation of methyl-directed mismatch repair is that the mispaired bases are recognized and bound by the MutS protein with MutL as a helper protein for binding. MutL also assists the MutH protein to bind, thereby forming the completed initiation complex of MutS, MutL and MutH. In the presence of ATP, there is evidence for translocation ofthe complex along the DNA forming alpha loops. At a d(GATC) site the MutH protein binds and nicks the unmethylated daughter DNA strand 5' to the d(G) (by recognizing the N6-d(A) methylation of the parental DNA strand which it is unable to cut). This completes the initiation of the repair system and allows the hydrolysis and resynthesis of the daughter DNA strand. MutH is a monomer of 25.5 kD in solution and contains a latent Mg2+-dependent endonuclease activity. Unmethylated DNA is nicked without any discrimination on one of the two strands and fully methylated DNA is resistant to cleavage by MutH even though the protein is able to bind the d(GATC) site. The structure of MutH was recently solved and compared to a group of restriction endonucleases that share a structural common core domain with similarly placed catalytic residues. The MutH protein is comprised of two major domains that are able to pivot and rotate with respect to one another. The cleft between the two domains is large enough for double-strand DNA to bind. This research started with the determination of the MutH structure before it was known. After crystallizing the protein and collecting several heavy atom data sets, it was found that the electron density maps were too discontinuous to trace the structure of the protein. Following that work, site-directed mutagenesis was performed on several areas of MutH based on the similarity of MutH and PvuII structural models. The aims were to identify DNA binding residues (in two flexible loop regions), to determine if MutH has the same mechanism for DNA binding and catalysis as PvuII (MutH histidines 112 and 115), and to localize the residues responsible for MutH stimulation by MutL (MutH C-terminal tail region). An in-vivoscreen based on the mutator phenotype was used to select for functionally defective MutH mutants. These bacteria accumulate mutations at a greater frequency than wild-type and this was monitored by selection on plates with rifampicin. Three MutH mutants were identified from this screen (K48A, G49A, and Δ214). They were purified and assayed for total activity and binding ability. Four other mutants with wild-type phenotypic screen results were also chosen to confirm they were not involved in any MutH function (D47A, H112A, H115A, and Δ224). No DNA binding residues (such as D47A) were identified in the two flexible loop regions of MutH, although similar loops in PvuII are involved in DNA binding. The purified D47A MutH protein showed wild-type biochemical activity. Instead, the lysine residue (K48) in the first flexible loop was found to function in catalysis together with the three presumed catalytic amino acids (Asp70, Glu77, and Lys79). This purified MutH protein (K48A) had wild-type binding ability but no endonuclease activity without MutL. In the presence of MutL, the K48A protein had only a three-fold reduction in endonuclease activity. This research has shown that MutL stimulates the wild-type MutH activity by 1000-fold. The wild-type MutH stimulation by MutL for binding was only shown to be 16-fold. The G49A MutH mutant interferes with the proper functioning of the protein but is not informative about the mechanism of action. The binding ability of this mutant was the same as wild-type and the endonuclease activity was down 30-fold with a 10-fold stimulation by MutL. The extra methyl group of the alanine may cause slight structural changes in the lysine 48 side chain that slows catalysis. The two histidines (H112 and H115) in MutH that are in a similar position as the two histidines (H84 and H85) in PvuII (that signal for DNA binding and catalysis) were changed to alanines, but had wild-type activity both in-vivo and in-vitro. These results indicate that the MutH signal for DNA binding and catalysis remains unknown. The two deletion mutations (MutHΔ224 and MutHΔ214) in the C-terminal end of the protein, localized the MutL stimulation region to five amino acids (Ala220, Leu221, Leu222, Ala223, and Arg224). Mutant MutHΔ224 had wild-type MutL stimulation activity, while MutHΔ214 showed no MutL stimulation. Another deletion mutant, MutHΔ119, from another laboratory was shown to have wild-type MutL stimulation also. This leaves one (or more) of the remaining five residues as important for MutL stimulation.

In previous work I described the purification and properties of the microtubule-based mechanochemical ATPase cytoplasmic dynein. Cytoplasmic dynein was found to produce force along microtubules in the direction corresponding to retrograde axonal transport. Cytoplasmic dynein has been identified in a variety of eukaryotes including yeast and human, and there is a growing body of evidence suggesting that this "molecular motor" is responsible for the transport of membranous organelles and mitotic chromosomes. The first part of this thesis investigates the molecular basis of microtubule-activation of the cytoplasmic dynein ATPase. By analogy with other mechanoenzymes, this appears to accelerate the rate-limiting step of the cross-bridge cycle, ADP release. Using limited proteolysis, site-directed antibodies, and N-terminal microsequencing, I identified the acidic C-termini of α and β-tubulin as the domains responsible for activation of the dynein ATPase. The second part of this thesis investigates the structure of the 74 kDa subunit of cytoplasmic dynein. The amino acid sequence deduced from cDNA clones predicts a 72,753 dalton polypeptide which includes the amino acid sequences of nine peptides determined by microsequencing. Northern analysis of rat brain poly(A) revealed an abundant 2.9 kb mRNA. However, PCR performed on first strand cDNA, together with the sequence of a partially matching tryptic peptide, indicate the existence of three isoforms. The C-terminal half is 26.4% identical and 47.7% similar to the product of the Chlamydomonas ODA6 gene, a 70 kDa subunit of flagellar outer arm dynein. Based on what is known about the Chlamydomonas70 kDa subunit, I suggest that the 74 kDa subunit is responsible for targeting cytoplasmic dynein to membranous organelles and kinetochores of mitotic chromosomes. The third part of this thesis investigates a 50 kDa polypeptide which co-purifies with cytoplasmic dynein on sucrose density gradients. Monoclonal antibodies were produced against the 50 kDa subunit and used to show that it is a component of a distinct 20S complex which contains additional subunits of 45 and 150 kDa. Moreover, like cytoplasmic dynein, the 50 kDa polypeptide localizes to kinetochores of metaphase chromosomes by light and electron microscopy. The 50 kDa-associated complex is reported to stimulate cytoplasmic dynein-mediated organelle motility in vitro. The complex is, therefore, a candidate for modulating cytoplasmic dynein activity during mitosis.

DNA is packaged within the cells' nucleus as a highly compact chromatin structure ranging between 100-400 nm fibers. The organization and alteration of this structure is mandatory in order to arbitrate DNA-mediated processes of the cell, including transcription, DNA replication, recombination and repair. Many different kinds of enzymes modify chromatin components and, in turn, regulate the accessibility of DNA. These multi-subunited enzymes have emerged as key regulators for several processes of the cell. Central to understanding how DNA-mediated processes are regulated is to comprehend the consequences of these modifications of chromatin, which lead to altered states of either activation or inactivation. One class of factors known to modify chromatin structure is the ATP-dependent chromatin remodeling enzymes. This class of enzymes encompasses evolutionarily conserved multi-subunited enzymes, which appear to function by using the energy of ATP hydrolysis to disrupt histone-DNA interactions. The prototype of ATP-dependent chromatin remodelers is the Saccharomyces cerevisiae SWI/SNF complex. The yeast SWI/SNF complex is required for the full functioning of several transcriptional activators and for the expression of a subset of yeast genes, a notable number being inducible and mitotic genes. The purified complex is comprised of the following eleven different polypeptides: Swi2p/Snf2p, Swi1p, Swi3p, Snf5p, Snf6p, Swp73p, Arp7p, Arp9p, Swp82p, Swp29p and Snf11 p. It has been established that a core of homologous subunits (Swi2p, Swi3p, Swp73p, Snf5p and the Arp proteins) is conserved among the SWI/SNF-related complexes from several organisms (yRSC, hSWI/SNF, hRSC, DrosophilaBrahma). However, the functional contribution of these polypeptides in the complexes for altering chromatin structure is largely unknown. In this study, biochemistry is used to examine the structure of the complex and function of individual subunits of the yeast SWI/SNF complex to understand better how these proteins are acting in concert to remodel chromatin. In addition, we examine a role for SWI/SNF complex in the process of DNA replication. The relative stoichiometry of the SWI/SNF complex subunits was determined by in vitrobiochemical studies. Co-immunoprecipitation has demonstrated that there is only one copy of Swi2p/Snf2p per complex. Subsequent radioactive labeling of the purified complex revealed that the complex contains one copy of each subunit per complex with the exception of Swi3p and Snf5p, which are present in two copies per complex. The subunit organization of SWI/SNF complex has been more clearly defined by determining direct subunit-subunit interactions in the complex. The Swi3p component has previously been shown to be critical for complex function in vivo and essential for the integrity of the complex in vitro, and this study demonstrates that Swi3p serves as a scaffolding protein that nucleates SWI/SNF complex assembly. In vitrobinding studies with Swi3p have revealed that Swi3p displays self-association, as well as direct interactions with the Swi2p, Snf5p, Swp73p, Swi1p and Snf6p members of the complex. The direct interactions of the yeast SWI/SNF subunits with transcriptional activators, thought to be important for yeast SWI/SNF targeting, were examined. In vitrobinding assays demonstrate that individual SWI/SNF subunits, Snf5p, Snf6p and Swi1p, and sub-complexes Swi2p/Swi3p and Swp73p/Swi3p can directly interact with specific domains of transcriptional activators of either the Swi5p zinc-finger DBD or VP16 acidic activation domain. This work begins to characterize the functional contribution of individual subunits, and cooperative sub-complexes that are critical for the SWI/SNF complex functional activities. The yeast SWI/SNF complex was investigated for the ability to playa role in DNA replication. Interestingly, plasmid stability assays reveal that minichromosomes that contain DNA replication origin ARS121 is weakened when the SWIISNF complex is non-functional. ARS121's SWI/SNF dependency is overcome by the over-expression of DNA replication regulatory protein, Cdc6p. Thus, this suggests SWI/SNF may either indirectly effect DNA replication by effecting the expression of Cdc6p, or has a redundant function with Cdc6p. In addition, several crippled derivatives of ARS1 acquire SWI/SNF dependence, and it is found that the SWI/SNF complex requires a transcriptional activation domain to enhance ARS1 function. These results reinforce the view that SWI/SNF play a role in two chromatin-mediated processes', transcription and DNA replication.

Members of the Argonaute family of proteins, which interact with small RNAs, are the key players of RNAi and other related pathways. The C. elegans genome encodes 27 members of the Argonaute family. During this thesis research, we sought to understand the functions of the members of this gene family in C. elegans. Among the Argonaute family members, rde-1 and alg-1/2have previously been shown to be essential for RNAi and development, respectively. In this work, we wanted to assign functions to the remaining members of this large family of proteins. Here, we describe the phenotype of 31 deletion alleles representing all of the previously uncharacterized Argonaute members. In addition to rde-1, our analysis revealed that two other Argonaute members csr-1 and prg-1 are also essential for development. csr-1 is partially required for RNAi, and essential for proper chromosome segregation. prg-1, a member of PIWI subfamily of Argonaute genes, exhibits reduced brood size and temperature-sensitive sterile phenotype, implicating that it is required for germline maintenance. Additionally, we showed that RDE-1 interacts with trigger-derived sense and antisense siRNAs (primary siRNAs) to initiate RNAi, while several other Argonaute proteins, SAGO-1, SAGO-2, and perhaps others, functioning redundantly, interact with amplified siRNAs (secondary siRNAs) to mediate downstream silencing. Moreover, our analysis uncovered that another member of Argonaute gene family, ergo-1, is essential for the endogenous RNAi pathway. Furthermore, we built an eight-fold Argonaute mutant, MAGO8, and analyzed its developmental phenotype and sensitivity to RNAi. Our analysis revealed that the genes deleted in the MAGO8 mutant function redundantly with each other, and are required for RNAi and the maintenance of the stem cell totipotency.

Myosin 10 is a vertebrate specific actin-based motor protein that is expressed in a variety of cell types. Cell biological evidences suggest that myosin 10 plays a role in cargo transport and filopodia extension. In order to fully appreciate these physiological processes, it is crucial to understand the motor property of myosin 10. However, little is known about its mechanoenzymatic characteristics. In vitro biochemical characterization of myosin 10 has been hindered by the low expression level of the protein in most tissues. In this study, we succeeded in obtaining sufficient amount of recombinant mammalian myosin 10 using the baculovirus expression system. The movement directionality of the heterologously expressed myosin 10 was determined to be plus end-directed by the in vitro motility assay with polarity-marked actin filament we developed. The result is consistent with the proposed physiological function of myosin 10 as a plus end-directed transporter inside filopodia. The duty ratio of myosin 10 was determined to be 0.6~0.7 by the enzyme kinetic analysis, suggesting that myosin 10 is a processive motor. Unexpectedly, we were unable to confirm the processive movement of dimeric myosin 10 along actin filaments in a single molecule study. The result does not support the proposed function of myosin 10 as a transporter. One possible explanation for this discrepancy is that the apparent nonprocessive nature of myosin 10 is important for generating sufficient force required for the intrafilopodial transport by working in concert with numbers of other myosin 10 molecules while not interfering with each other. Altogether, the present study provided qualitative and quantitative biochemical evidences for the better understanding of the motor property of myosin 10 and of the biological processes in which it is involved. Finally, a general molecular mechanism of myosin motors behind the movement directionality and the processivity is discussed based on our results together with the currently available experimental evidences. The validity of the widely accepted ‘leverarm hypothesis’ is reexamined.