Interactions between the quality control ubiquitin ligase CHIP and ubiquitin conjugating enzymes

The 2.9 ？ crystal structure of the CHIP U-box domain complexed with UbcH5a shows that CHIP binds to UbcH5 and Ubc13 through similar specificity determinants, including a key S-P-A motif on the E2 enzymes. The determinants make different relative contributions to the overall interactions between CHIP and the two E2 enzymes. CHIP undergoes auto-ubiquitination by UbcH5 but not by Ubc13-Uev1a. Instead, CHIP drives the formation of unanchored polyubiquitin by Ubc13-Uev1a. CHIP also interacts productively with the class III E2 enzyme Ube2e2, in which the UbcH5- and Ubc13-binding specificity determinants are highly conserved.The CHIP:UbcH5a structure emphasizes the importance of specificity determinants located on the long loops and central helix of the CHIP U-box, and on the N-terminal helix and loops L4 and L7 of its cognate E2 enzymes. The S-P-A motif and other specificity determinants define the set of cognate E2 enzymes for CHIP, which likely includes several Class III E2 enzymes. CHIP's interactions with UbcH5, Ube2e2 and Ubc13-Uev1a are consistent with the notion that Ubc13-Uev1a may work sequentially with other E2 enzymes to carry out K63-linked polyubiquitination of CHIP substrates.Ubiquitination is a key posttranslational modification that is involved in most aspects of cellular homeostasis, signalling and regulation. In ubiquitination, sequential action of E1 (ubiquitin-activating), E2 (ubiquitin-conjugating) and E3 (ubiquitin ligase) proteins act sequentially to promote attachment of the 76 amino-acid polypeptide ubiquitin to a substrate protein[1,2]. Ubiquitin is attached to the substrate through an isopeptide bond between the C-terminus of ubiquitin and the ε-amino group of a substrate lysine. E1 enzymes transfer ubiquitin to the active site cysteine of an E2 enzyme [3]. E2 enzymes, in turn, bind to E3 ligases, which contain either HECT domains or a member of the structurally similar RING, PHD-like and U-box domain superfamily [4-6]. E3 ligases also contain o