Cks1 proteasomal turnover is a predominant mode of regulation in breast cancer cells: Role of key tyrosines and lysines

Constitutive levels of Cks1 protein are very high in mammary carcinoma tissue and in breast tumor cell lines. However, despite being transcribed at relatively high levels, Cks1 protein is very low in normal mammary tissue. Also, basal Cks1 is barely detectable in primary human mammary epithelial cells (HMECs). Epoximicin, a proteasome inhibitor, induced detectable endogenous Cks1 in HMECs, and upregulated it above the basal level in MCF-7 breast cancer cells. Interestingly, transiently transfected Cks1 is remarkably unstable and accumulates only upon proteasomal blockade in multiple cell lines even when driven by the strong CMV promoter-enhancer. We examined the stability of site-directed Cks1 mutants in order to identify the structural determinants of its turnover in cancer cells. Since protein turnover is regulated by phosphorylation, and phosphoproteomic studies reveal phosphorylated tyrosines in Cks1, we replaced its five conserved tyrosines (Y) with phenylalanine (F), both individually and in combinations. We find that like wild-type, all transiently transfected mutant Cks1 vectors, even when driven by the CMV promoter-enhancer, expressed detectable protein only in cells treated with epoximicin. However, turnover of the Y8F, Y12F and Y19F Cks1 mutants was more rapid than that of wild-type, Y7F and Y57F. Since lysines are modified by ubiquitination or acetylation we also examined the consequences of lysine to arginine (K-R) substitutions on Cks1 proteasomal turnover. We found that the individual mutations K4R, K26R, K30R, and K34R slowed Cks1 turnover, while the K79R mutation or the combined mutation K75-76-78-79R increased turnover. Taken together, regulation of Cks1 protein stability is crucially dependent on specific tyrosine and lysine residues which are potential sites for post-translational modifications.