The spatio-temporal organization and dynamic behavior of microtubules accurately react to cellular needs during intracellular transport, signal transduction, growth, division, and differentiation. The cell generates centrosomal microtubules de novo with the help of γ-tubulin complexes (γTuRCs). The post-translational modifications fine-tune microtubule nucleation by targeting the proteins, interacting with γTuRCs. However, the exact signaling pathways, regulating centrosomal microtubule nucleation, remain mostly unknown. In the presented thesis, we functionally characterized protein tyrosine phosphatase SHP-1 and E3 UFM-protein ligase 1 (UFL1) with its interacting protein CDK5RAP3 (C53) in the regulation of centrosomal microtubule nucleation. We also elucidated the role of actin regulatory protein profilin 1 in this process. We found that SHP-1 formed complexes with γTuRC proteins and negatively regulated microtubule nucleation by modulating the amount of γ-tubulin/γTuRC at the centrosomes in bone marrow-derived mast cells (BMMCs). We suggested a novel mechanism with centrosomal tyrosine-phosphorylated Syk kinase, targeted by SHP-1 during Ag-induced BMMCs activation, regulating microtubules. We showed for the first time that UFL1/C53 protein complex is involved in the regulation of microtubule...

Mutations in profilin 1 (PFN1) cause amyotrophic lateral sclerosis (ALS), a fatal neurodegenerative disease that targets motor neurons. PFN1 is a 15 kDa protein that is best known for its role in actin dynamics. However, little is known about the pathological mechanisms of PFN1 in ALS. In this dissertation, it is demonstrated that certain familial ALS-linked mutations severely destabilize the native conformation of PFN1 in vitro and cause accelerated turnover of the PFN1 protein in neuronal cells. This mutation-induced destabilization can account for the high propensity of ALS-linked variants to aggregate and also provides rationale for their reported functional defects in cell-based assays. The source of this destabilization is illuminated by the crystal structures of several PFN1 proteins, revealing an expanded cavity near the protein core of one ALS variant and predicting a non-surface exposed cavity in another. Functional biochemical experiments point to abnormalities in actin filament nucleation and elongation caused by PFN1 mutants. In HeLa cells, PFN1 is essential for the generation of actin-rich filopodia and expression of mutant PFN1 alters filopodia density further supporting a pathogenesis mechanism involving actin cytoskeleton. Taken together, this dissertation infers that the pathogenesis of ALS due to mutations in PFN1 can be mediated at least by two possibly related mechanisms, a destabilization of the native PFN1 structure and an impact on the actin assembly processes.