Ubiquitinome profiling identifies USP13-CMAS axis as critical regulator of hair follicular melanogenesis via K48-linked polyubiquitination.

The ubiquitin-proteasome system critically regulates melanogenesis through post-translational modifications. However, the specific deubiquitination substrates involved in this regulation remain poorly characterized. This study employed multi-omics integration and functional validation to decipher the role of USP13 in melanocyte (MC) biology. Through quantitative melanin assays and apoptosis/proliferation analyses, we established the dual functionality of USP13 in promoting MC pigmentation and survival. Systematic proteome-ubiquitinome analysis of USP13-overexpressing MCs identified 23 high-confidence substrates. Cytidine monophosphate N-acetylneuraminic acid synthetase (CMAS) was recognized as a key mediator. Mechanistically, Co-IP studies demonstratedthat USP13 stabilizes CMAS by specifically cleaving its K48-linked polyubiquitin chains, thereby enhancing melanogenic capacity. Furthermore, a novel transcriptional regulatory axis was identified, in which FOXO4 represses USP13 expression through direct promoter interaction, as demonstrated by luciferase reporter and EMSA assays. This study expands the substrate landscape of USP13 but also delineates a novel FOXO4-USP13-CMAS axis coordinating transcriptional and post-translational control of melanin biosynthesis. The results provide new therapeutic targets for pigmentary disorders.