Cell polarity is fundamental to numerous cellular processes including migration, molecular transport, and cell division. The establishment and organization of polarity is crucial to the maintenance of cellular homeostasis in mammalian systems. Deregulation of cell polarity is observed in disease states, including cancer. Numb is an adaptor protein that functions in regulating endocytic trafficking events. Numb was originally identified in Drosophila as an asymmetrically localized cell fate determinant, and was subsequently found to be conserved in vertebrates. In mammalian polarized epithelial cells, Numb is distributed asymmetrically along the basolateral membrane domain. The work herein describes phosphorylation of Numb by the Par complex protein, atypical Protein Kinase C (aPKC), as a means of regulating membrane localization and asymmetric distribution of Numb. A mutant of Numb that cannot be phosphorylated by aPKC accumulates on the plasma membrane and localizes to both apical and basolateral membranes. In aPKC-depleted cells, endogenous Numb is unable to achieve polarized distribution and localizes around the entire cell cortex. We demonstrate that this mechanism is conserved in Drosophila as mutation of the corresponding phosphorylation sites disrupts Numb asymmetric localization in dividing sensory organ precursor cells. In polarized epithelial cells, one function of Numb is to promote epithelial morphology when cells are challenged with external stimuli that disrupt cell-cell adhesion. For example, depletion of Numb results in enhanced sensitivity of cells to lose cell-cell contacts when treated with calcium chelating agents. Loss of Numb potentiates hepatocyte growth factor (HGF)-induced lamellipodia formation and cell dispersal – early steps in epithelial-mesenchymal transition (EMT). In Numb-depleted cells, Rac1-GTP loading is enhanced, which corresponds with increased rate in loss of cell-cell adhesion and increased lamellipodia formation, following depletion of extracellular calcium and HGF stimulation, respectively. Together, this work identifies a mechanism that regulates polarized distribution of Numb and provides insight into its function in polarized epithelial cells.

Histone phosphorylation is often a direct outcome of activated intracellular signaling pathways, and functions to translate extracellular signals into appropriate biological outputs such as changes in gene expression. Growth factors and cellular stress trigger rapid and transient expression of immediate-early genes (such as c-fos, c-jun) in mammalian cells, and their induction strongly correlates with a transient phosphorylation of S10 and S28 on histone H3. While many signaling cascades that lead to H3 phosphorylation have been mapped out, mechanistic details of the downstream events and how H3 phosphorylation contributes to transcriptional activation are still poorly defined. To investigate the direct effects of H3 phosphorylation on transcription, we targeted the H3 kinase MSK1 to endogenous c-fos promoter, and found that this is sufficient to activate its expression. Moreover, targeting MSK1 to the tissue-specific -globin gene induces H3S28 phosphorylation and reactivates expression of this polycomb-silenced gene. Mechanistically, H3S28 phosphorylation not only disrupts binding of polycomb repressive complexes, but also induces a methyl-acetylation switch of the adjacent K27 residue. This provides the first indication that H3 phosphorylation is involved in antagonizing polycomb silencing. To further identify post-translational modifications (PTMs) that function together with MSK1-mediated H3 phosphorylation, I developed a novel nucleosome purification approach called Biotinylation-assisted Isolation of CO-modified Nucleosomes (BICON). This technique combines in vivo biotinylation by BirA and H3 phosphorylation by MSK1, allowing enrichment of phosphorylated nucleosomes using streptavidin. I found that MSK1-phosphorylated nucleosomes are hyper-acetylated on H3 and H4, and importantly, I identified a trans-tail crosstalk between H3 phosphorylation and H4 acetylation on K12. This proof-of-principle study demonstrates that BICON can be further adapted to study PTMs and crosstalks associated with other histone-modifying enzymes. Taken together, work described in this thesis shows that histone H3 phosphorylation can initiate additional PTM changes on other residues within the nucleosome, and such crosstalk plays an important role in regulating gene expression.

Histone phosphorylation is often a direct outcome of activated intracellular signaling pathways, and functions to translate extracellular signals into appropriate biological outputs such as changes in gene expression. Growth factors and cellular stress trigger rapid and transient expression of immediate-early genes (such as c-fos, c-jun) in mammalian cells, and their induction strongly correlates with a transient phosphorylation of S10 and S28 on histone H3. While many signaling cascades that lead to H3 phosphorylation have been mapped out, mechanistic details of the downstream events and how H3 phosphorylation contributes to transcriptional activation are still poorly defined. To investigate the direct effects of H3 phosphorylation on transcription, we targeted the H3 kinase MSK1 to endogenous c-fos promoter, and found that this is sufficient to activate its expression. Moreover, targeting MSK1 to the tissue-specific -globin gene induces H3S28 phosphorylation and reactivates expression of this polycomb-silenced gene. Mechanistically, H3S28 phosphorylation not only disrupts binding of polycomb repressive complexes, but also induces a methyl-acetylation switch of the adjacent K27 residue. This provides the first indication that H3 phosphorylation is involved in antagonizing polycomb silencing. To further identify post-translational modifications (PTMs) that function together with MSK1-mediated H3 phosphorylation, I developed a novel nucleosome purification approach called Biotinylation-assisted Isolation of CO-modified Nucleosomes (BICON). This technique combines in vivo biotinylation by BirA and H3 phosphorylation by MSK1, allowing enrichment of phosphorylated nucleosomes using streptavidin. I found that MSK1-phosphorylated nucleosomes are hyper-acetylated on H3 and H4, and importantly, I identified a trans-tail crosstalk between H3 phosphorylation and H4 acetylation on K12. This proof-of-principle study demonstrates that BICON can be further adapted to study PTMs and crosstalks associated with other histone-modifying enzymes. Taken together, work described in this thesis shows that histone H3 phosphorylation can initiate additional PTM changes on other residues within the nucleosome, and such crosstalk plays an important role in regulating gene expression.

The precise mechanism regulating focal adhesion disassembly has yet to be elucidated. Recently, we have implicated the Ste20-like kinase SLK in mediating efficient focal adhesion turnover and cell migration in a Rac-1 and FAK-dependent manner. Although an indirect association of this kinase with the microtubule network has been determined, the exact involvement of SLK in the disassembly of the adhesion complex remains unclear. With the identification of the focal adhesion protein paxillin as a substrate of SLK, we show that SLK regulates adhesion turnover through its phosphorylation at S250. Mutation of S250 to a threonine residue ablates SLK phosphorylation of paxillin in vitro and results in reduced adhesion turnover and migration in vivo. Additionally, our studies demonstrate that overexpression of the paxillin S250T mutation prevents the redistribution of paxillin to the membrane ruffle in migrating cells. The complete loss of polyubiquitylation in the S250T mutant, combined with no observed reduction in S250T protein expression, suggests that S250 phosphorylation is required for a ubiquitin-mediated modification that regulates paxillin redistribution within the cell. Moreover, we show that phosphorylation of S250 is required for paxillin to interact with FAK. An observed accumulation of phospho-FAKY397 in cells overexpressing the paxillin S250T mutant suggests that phosphorylation of S250 is involved in regulating FAK-dependent focal adhesion dynamics. Consequently, our data suggests that SLK regulates adhesion turnover through the phosphorylation of paxillin at S250.

Thesis (Ph. D.)--University of Hawaii at Manoa, 1995. / Includes bibliographical references (leaves 76-93). / Microfiche. / vii, 93 leaves, bound ill. (some col.) 29 cm

Tuberculosis, caused by the intracellular pathogen Mycobacterium tuberculosis, is one of the most prevalent infectious diseases in our world today. In order to survive within the host the bacteria need to sense and respond to changes in the environment; however, signal transduction in this bacterium is poorly understood. PknB is a serine/threonine kinase essential for the in vitro survival of M. tuberculosis and therefore a potential drug target against the bacteria. It is the goal of the current study to elucidate downstream substrates of PknB. We have found that PknB shares in vitro substrates with another serine/threonine kinase, PknH, implying the potential complexity of the signaling pathways in the bacteria. We have also provided the first description of the coupling between serine/threonine kinases PknB and PknH with a two-component system response regulator DevR, and further proposed Ser/Thr phosphorylation as the negative regulator of DevR transcription activator activity based on LC-MS/MS analysis. Finally, we have identified a previously unknown phosphoprotein glyceraldehyde 3-phosphate dehydrogenase encoded by the ORF Rv1436, which demonstrates autophosphorylation activity and which phosphorylation is independent of PknB. Overall, the current study has contributed to advance our understanding of the signal transduction pathways and phosphoproteome in Mycobacterium tuberculosis.

S. cerevisiae have developed the ability to forage for nutrients when presented with conditions of starvation. This dimorphic adaptation is particularly noticeable when yeast are subject to nitrogen depravation and has been termed filamentous growth, as cells form filament-like projections away from the center of the colony. The regulation of this response is under the control of the well-characterized MAPK and cAMP pathways. Previous work showed that Cdk8p phosphorylated a key transcriptional activator of the filamentous response, Ste12p, and subsequently targeted the factor for degradation under conditions of limiting nitrogen. Data presented in this thesis suggests that Cdk8p is regulated by another kinase, Tpk2p. In vitro kinase assays demonstrate that Tpk2p directly phosphorylates Cdk8p on residue Thr37, leading to the destabilization of Cdk8p after growth for 4 hours in SLAD media. Lack of phosphorylation on Thr37 yields a hypo-hypofilamentous phenotype, whereas a phospho-mimic mutant, T37E displays a filamentous hyper-filamentous phenotype.

Thesis (Ph. D.)--Ohio State University, 2007. / Title from first page of PDF file. Includes bibliographical references (p. 137-154).

Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.

Thesis (Ph. D.)--University of Washington, 2007. / Vita. Includes bibliographical references (leaves 89-100).