Deciphering Site-Specific Regulatory Networks of the Kinesin Protein KIF21A Through Integrative Phosphoproteomic Analysis.

KIF21A, a member of the Kinesin-4 family of motor proteins, is involved in the regulation of microtubule dynamics and intracellular transport, with emerging evidence suggesting its potential role in cancer progression. In this study, we performed an integrative analysis of over 3825 human phosphoproteomics studies to characterize site-specific phosphorylation of KIF21A. Three predominant phosphosites were identified in KIF21A (S853, S1212, and S1239) with the highest detection frequency across phosphoproteomics studies, and were analyzed for co-regulation patterns to identify potential kinase associations and functional networks. Phosphosite S853 showed a strong association with cytoskeletal organization and cortical microtubule stabilization complexes (CMSCs) components, including KANK1 (S186), PHLDB2 (S513, S42) and CLASP1 (S600, S572, S646), indicating its role in cytoskeletal organization. Upstream kinase analysis identified potential regulators, such as PAK2, RPS6KA1/A3, RPS6KB1, CHEK1/2 and CDK18/16 with site-specific variability in their associations with KIF21A predominant sites. Interestingly, phosphosite-specific correlation analysis between KIF21A and candidate kinases revealed that the KIF21A S1239 phosphosite exhibited tumor-specific correlations with CDK18 across multiple cancer types. Functional enrichment revealed that co-regulated phosphoproteins were involved in cytoskeleton regulation, cell cycle regulation, and carcinogenesis. Pan-cancer analysis demonstrated dysregulated expression of KIF21A in multiple tumor types, with stage-associated upregulation in selected cancers. Gene-level validation further supported these findings, showing consistent positive correlations between KIF21A and key regulators such as CTNND1 and PTK2, as well as other cytoskeleton and cancer-associated genes. Overall, this study highlights site-specific phosphorylation as a key regulatory mechanism of KIF21A and suggests its involvement in cytoskeleton-associated signaling networks in cancer.