Monoubiquitination empowers ubiquitin chain elongation

Abstract

Ubiquitination often generates lysine 48-linked polyubiquitin chains that signal proteolytic destruction of the protein target. A significant subset of ubiquitination proceeds by a priming/extending mechanism, in which a substrate is first mono-ubiquitinated with a priming E2 conjugating enzyme or a set of E3 ARIH/E2 enzymes specific for priming. This is then followed by ubiquitin chain extension catalyzed by an E2 enzyme capable of elongation. This report provides further insights into the priming/extending mechanism. We employed reconstituted ubiquitination systems of substrates CK1α and β-catenin by Cullin-RING E3 ubiquitin ligases (CRLs) CRL4CRBN and CRL1βTrCP, respectively, in the presence of priming E2 UbcH5c and elongating E2 Cdc34b. We have established a new “Apyrase chase” strategy that uncouples priming from chain elongation, which allows accurate measurement of the decay rates of the ubiquitinated substrate with a defined chain length. Our work has revealed highly robust turnover of mono-ubiquitinated β-catenin that empowers efficient polyubiquitination. The results of competition experiments suggest that the interactions between the ubiquitinated β-catenin and CRL1βTrCP are highly dynamic. Moreover, ubiquitination of the ubiquitin-modified β-catenin appeared more resistant to inhibition by competitors than the unmodified substrate, suggesting tighter binding with CRL1βTrCP. These findings support a role for conjugated ubiquitin in enhancing interactions with E3.