The Catalytically Inactive Mutation of the Ubiquitin-Conjugating Enzyme CDC34 Affects its Stability and Cell Proliferation

Xun Liu, Yang Zhang, Zhanhong Hu, Qian Li, Lu Yang, Guoqiang Xu

The ubiquitin proteasome system (UPS) plays important roles in the regulation of protein stability, localization, and activity. A myriad of studies have focused on the functions of ubiquitin ligases E3s and deubiquitinating enzymes DUBs due to their specificity in the recognition of downstream substrates. However, the roles of the most ubiquitin-conjugating enzymes E2s are not completely understood except that they transport the activated ubiquitin and form E2-E3 protein complexes. Ubiquitin-conjugating enzyme CDC34 can promote the degradation of downstream targets through the UPS whereas its non-catalytic functions are still elusive. Here, we find that mutation of the catalytically active cysteine to serine (C93S) results in the reduced ubiquitination, increased stability, and attenuated degradation rate of CDC34. Through semi-quantitative proteomics, we identify the CDC34-interacting proteins and discover that the wild-type and mutant proteins have many differentially interacted proteins. Detailed examination finds that some of them are involved in the regulation of gene expression, cell growth, and cell proliferation. Cell proliferation assay reveals that both the wild-type and C93S proteins affect the proliferation of a cancer cell line. Database analyses show that CDC34 mRNA is highly expressed in multiple cancers, which is correlated with the reduced patient survival rate. This work may help to elucidate the enzymatic and non-enzymatic functions of this protein and might provide additional insights for drug discovery targeting E2s.