The APC11 RING-H2 finger mediates E2-dependent ubiquitination

Polyubiquitination marks proteins for degradation by the 26S proteasome and is carried out by a cascade of enzymes that includes ubiquitin-activating enzymes (E1s), ubiquitin-conjugating enzymes (E2s), and ubiquitin Ligases ( E3s). The anaphase-promoting complex or cyclosome (APC/C) comprises a multi subunit ubiquitin ligase that mediates mitotic progression. Here, we provid e evidence that the Saccharomyces cerevisiae RING-H2 finger protein Apc11 d efines the minimal ubiquitin ligase activity of the APC. We found that the integrity of the Apc11p RING-H2 finger was essential for budding yeast cell viability, Using purified, recombinant proteins we showed that Apc11p inte racted directly with the Ubc4 ubiquitin conjugating enzyme (E2). Furthermor e, purified Apc11p was capable of mediating E1- and E2-dependent ubiquitina tion of protein substrates, including Clb2p, in vitro. The ability of Apc11 p to act as an E3 was dependent on the integrity of the RING-H2 finger, but did not require the presence of the cullin-like APC subunit Apc2p. We sugg est that Apc11p is responsible for recruiting E2s to the APC and for mediat ing the subsequent transfer of ubiquitin to APC substrates in vivo.