Mastery of MAST3 Nonkinase Domain Phosphosites in Regulating Cytoskeletal Organization

Microtubule-associated serine/threonine-protein kinase 3 (MAST3) is a member of the MAST kinase family implicated in neuronal and immune pathways and is predicted to associate with cytoskeletal regulation. However, insights into its functional role in cytoskeletal organization remain unexplored. In this study, we performed a large-scale phosphoproteomic analysis of MAST3 using 562 datasets to delineate its functional network. We identified four predominant phosphosites, S134, S146, S792, and S793, based on the frequency of detection and differential regulation, with S134 and S146 localized within the Domain of Unknown Function domain, a noncatalytic region. These phosphosites exhibited distinct coregulatory profiles, suggesting regulation through noncatalytic domains. Coregulated phosphosites were enriched for cytoskeleton-associated functions, including actin filament organization, microtubule organization, and spindle assembly. Additionally, predicted downstream substrates such as KIF15, EPB41L1, CP110, and HNRNPU, and binary interactors including LMNA, CKAP4, and CAMSAP2, further support the involvement of MAST3 in cytoskeletal regulation. The convergence of these cytoskeletal partners across phosphosites, substrates, and interactors suggests that MAST3 may act as a key modulator of cytoskeletal organization through phosphorylation-dependent protein-protein interactions. Notably, frequent phosphorylation of S146 across cancer types points to a potential tumor-specific regulatory role. Together, these findings provide the first systems-level insight into the role of MAST3 in cytoskeletal regulation and disease relevance.