Thesis (M.S.)--The University of North Carolina at Greensboro, 2008. / Directed by Paul Steimle; submitted to the Dept. of Biology. Title from PDF t.p. (viewed Mar. 19, 2010). Includes bibliographical references (p. 30-31).

Dictyostelium DdINCENP is a chromosomal passenger protein associated with centromeres, the spindle midzone and poles during mitosis and the cleavage furrow during cytokinesis. Disruption of the single DdINCENP gene revealed important roles for this protein in mitosis and cytokinesis. DdINCENP null cells lack a robust spindle midzone and are hypersensitive to microtubule depolymerizing drugs suggesting that their spindles may not be stable. Furthermore DdCP224, a protein homologous to the microtubule-stabilizing protein TOGp/XMAP215, was absent from the spindle midzone of DdINCENP null cells. Overexpression of DdCP224 rescued the weak spindle midzone defect of DdINCENP null cells. While not required for the localization of the myosin II contractile ring and subsequent formation of a cleavage furrow, DdINCENP is important for the abscission of daughter cells at the end of cytokinesis. The localization of DdINCENP at the cleavage furrow is modulated by myosin II. Loss of myosin II restricted the localization of DdINCENP to a narrow zone at the cleavage furrow. Kif12, a homolog of mitotic kinesin like protein (MKLP), was essential for relocalization of DdINCENP from the central spindle to the cleavage furrow. Furthermore, Kif12 was also localized at the cortex of the cleavage furrow and its localization during cytokinesis closely resembled that of DdINCENP, suggesting a possible interaction between them. The correct localization of DdINCENP during cytokinesis also required its N-terminal sequence. DdINCENP1-500 was found at the cleavage furrow and interacted with the actin cytoskeleton. Domain analysis of DdINCENP also revealed that its DdINCENP1-500 was sufficient to rescue the weak spindle defect of DdINCENP null cells. / text

Protein mechanical responses play a critical role in a wide variety of biological phenomena, impacting events as diverse as muscle contraction and stem cell differentiation. Recent advances in both experimental and computational techniques have provided the opportunity to explore protein constitutive properties at the molecular level. However, despite these advances many questions remain about how proteins respond to applied mechanical forces, particularly as a function of load magnitude. In order to address these questions, relatively simple helical structures were computationally tested to determine the mechanisms and kinetics of unfolding at a range of physiologically relevant load magnitudes. Atomically detailed constant force molecular dynamics simulations combined with the Milestoning kinetic analysis framework revealed that the mean first passage time (MFPT) of the initiation of unfolding of long (~16nm) isolated helical domains was a non-monotonic function of the magnitude of applied tensile load. The unfolding kinetics followed a profile ranging from 2.5ns (0pN) to a peak of 3.75ns (20pN) with a decreasing MFPT beyond 40pN reflected by an MFPT of 1ns for 100pN. The application of the Milestoning framework with a coarse-grained network analysis approach revealed that intermediate loads (15pN-25pN) retarded unfolding by opening additional, slower unfolding pathways through non-native [pi]-helical conformations. Analysis of coiled-coil helical pairs revealed that the presence of the second neighboring helix delayed unfolding initiation by a factor of 20, with calculated MFPTs ranging from 55ns (0pN) to 85ns (25pN per helix) to 20ns (100pN per helix). The stability of the coiled-coil domains relative to the isolated helix was shown to reflect a decreased propensity to break flexibility restraining intra-helix hydrogen bonds, thereby delaying [psi] backbone dihedral angle rotation and unfolding. These results show for the first time a statistically determined profile of unfolding kinetics for an atomically detailed protein that is non-monotonic with respect to load caused by a change in the unfolding mechanism with load. Together, the methods introduced for analyzing the mechanical response of proteins as well as the timescales determined for the initiation of unfolding provide a framework for the determination of the constitutive properties of proteins and non-biological polymers with more complicated geometries. / text

Chemotaxis is a process whereby single- and multi-cellular organisms migrate in response to external chemical stimuli. This directed cell movement is regulated by complex signaling pathways and is implicated in embryonic development, immune response, and the metastasis of cancer cells. Dictyostelium discoideum, social amoebae with the ability to migrate and aggregate in response to chemoattractants such as cAMP, have been used as a model system to study chemotaxis. Preliminary research suggests that protein kinase (PKA) is involved in some of the signaling pathways that regulate chemotaxis. The role of PKA in chemotaxis was investigated, first, by characterizing the phenotype of PKA null cells using established cell biological and biochemical assays. Furthermore, spatiotemporal regulation of critical cytoskeletal proteins was probed in wild-type and PKA null cells using confocal fluorescence microscopy, indicating misregulation of both F-actin and Myosin II in pkaC- and pkaR- cells. Finally, preliminary work was done to lay the groundwork for experiments exploring possible PKA targets mediating TORC2 function in chemotaxis.

Background and aims: It is known that smooth muscle possesses substantial mechanical plasticity in that it is able to adapt to large changes in length without compromising its ability to generate force. It is believed that structural malleability of the contractile apparatus underlies this plasticity. There is strong evidence suggesting that myosin thick filaments of the muscle are relatively labile and their length in vivo is determined by the equilibrium between monomeric and filamentous myosin. The equilibrium in turn is governed by the state of phosphorylation of the 20-kD regulatory myosin light chain (MLC20, or RLC). It is known that phosphorylation of the myosin light chain favors formation of the filaments; it is not known how the light chain phosphorylation affects the lability of the filaments. The major aim of this thesis was to measure the mechanical integrity of the filaments formed from purified myosin molecules from bovine airway smooth muscle, and to determine whether the integrity was influenced by phosphorylation of the myosin light chain. Methods: Myosin was purified from bovine trachealis to form filaments, in ATP containing zero-calcium solution during a slow dialysis that gradually reduced the ionic strength. Sufficient myosin light chain kinase and phosphatase, as well as calmodulin, were retained after the myosin purification and this enabled phosphorylation of RLC within 20-40 s after addition of calcium to the filament suspension. The phosphorylated and non-phosphorylated filaments were then partially disassembled by ultrasonification. The extent of filament disintegration was visualized and quantified by atomic force microscopy. Results: RLC phosphorylation reduced the diameter of the filaments and rendered the filaments more resistant to ultrasonic agitation. Electron microscopy revealed a similar reduction in filament diameter in intact smooth muscle when the cells were activated. Conclusion: Our results suggest that RLC phosphorylation is a key regulatory step in modifying the structural properties of myosin filaments in smooth muscle, where formation and dissolution of the filaments are required in the cells’ adaptation to different cell length.

The class I myosins are an ubiquitous family of non-filamentous, single-headed actin-binding motor proteins. The objective of this study was to identify the light chain composition of the short-tailed Dictyostelium class I myosins, MyoIA and MyoIE. Flag-tagged MyoIA head-neck and MyoIE head-neck constructs were generated and expressed in Dicyostelium discoidem. The MyoIA and MyoIE head-neck constructs both co-purified with a 17-kDa protein that reacted with an anti-calmodulin antibody and exhibited a mobility shift on SDS gels in the presence of calcium. Mass spectrometry analysis confirmed that the light chain bound to MyoIA and MyoIE was calmodulin. The finding that the short-tailed class I Dictyostelium myosins use the generic calcium-binding protein calmodulin as a light chain contrasts with previous work showing that the long-tailed Dictyostelium class I myosins MyoIB, MyoIC, and MyoID each bind a unique, specialized light chain called MlcB, MlcC, and MlcD, respectively. Despite having a calmodulin light chain, calcium did not affect the actin-activated Mg-ATPase activities of MyoIA or MyoIE. The p21-activated kinases (PAKs) are serine-threonine protein kinases that are activated by the small GTPases Cdc42 and Rac. PAKs phosphorylate a site in the motor domain of Dictyostelium class I myosins that is required for myosin activity. Studies were carried out to determine whether Dictyostelium RacB, which is known to bind to and activate Dictyostelium PAKs, promotes the phosphorylation of MyoID in vivo. A vector that expresses a constitutively active RacB under the control of a doxycycline-inducible promoter was created and transformed into Dictyostelium cells. Immunostaining demonstrated that the constitutively active RacB increased actin filament formation in AX3 cells by ~3-fold but by only ~1.5-fold in PakB-null cells. A rabbit polyclonal antibody against the MyoID tail was made. An anti-phospho antibody raised against a phosphorylated peptide corresponding to the MyoID TEDS site was tested and found to specifically recognize purified phosphorylation MyoIA and MyoID. The anti-phospho antibody did not detect phosphorylated MyoIA or MyoID in crude Dictyostelium cell extracts or in immunoprecipitates prepared using the anti-MyoID antibody. Further work is needed to improve the specificity of the anti-phospho MyoID antibody. / Thesis (Master, Biochemistry) -- Queen's University, 2009-09-24 19:51:55.032

The purpose of this investigation was to determine the effects of a 12-week progressive resistance training program (PRT) on single myofiber calcium sensitivity in older women. Five healthy older women between the ages of 67-82 with a mean age of 72.8±2.7 yr. participated in this study. The training regimen consisted of bilateral isotonic knee extensions. The subjects performed 3 sets, the first two sets consisted of 10 repetitions and the last set was performed to volitional exhaustion at 80% of their 1RM 3 days/week. Muscle biopsies of the vastus lateralis were obtained and single muscle fibers were isolated. The fibers were mounted and fiber length and diameter were determined. The experimental sequence for each fiber was the determination of maximal isometric tension (PO) at pCa 4.5 (pCa = -log [Ca 2+]), and then subsequent activations of the fiber submaximally with free Ca 2+ concentrations of pCa 6.8, 6.5, 6.2, 6.0, 5.8, 5.5, 5.2, 5.0, and 4.7. Due to the small sample size of the myosin heavy chain (MHC) type II single fibers that were studied, only the MHC type I fibers were included for analysis. The MHC type I single fiber diameter increased significantly (p<0.05) from 90.47±3.90 µm to 102.47±2.27 gm pre-to-post PRT, respectively. The Po increased approximately 22% (p<0.05) in the MHC type I fibers pre- to-post PRT. The mean MHC type I fiber Ca 2+ activation threshold increased (p<0.05) from pCa 6.83±0.02 to pCa 6.91±0.01 preto-post PRT. In addition, the mean half-maximal activation of the type I fibers increased (p<0.05) with PRT (5.50±0.02 and 5.70±0.03, pre and post, respectively. The slope of the tension-pCa relationship below (n2) and above (ni) half-maximal activation were also examined to predict molecular cooperativity during cross-bridge interaction. The slope of the Hill plot for n1 did not change significantly with the PRT. However, the slope of the Hill plot for n2 demonstrated a significant increase (p<0.05) from 1.70±0.11 to 2.43±0.09 pre-to-post PRT. In conclusion, the results of this investigation indicate that myofibril Ca 2+ sensitivity and activation properties appear to exhibit a significant role in the mechanisms involved with skeletal muscle adaptability in older women following PRT. / School of Physical Education

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Thesis (Ph. D.)--University of Washington, 1997. / Vita. Includes bibliographical references (leaves [67]-75).