A heterotypic assembly mechanism regulates CHIP E3 ligase activity.

CHIP (C‐terminus of Hsc70‐interacting protein) and its worm ortholog CHN‐1 are E3 ubiquitin ligases that link the chaperone system with the ubiquitin‐proteasome system (UPS). CHN‐1 can cooperate with UFD‐2, another E3 ligase, to accelerate ubiquitin chain formation; however, the basis for the high processivity of this E3s set has remained obscure. Here, we studied the molecular mechanism and function of the CHN‐1–UFD‐2 complex in Caenorhabditis elegans. Our data show that UFD‐2 binding promotes the cooperation between CHN‐1 and ubiquitin‐conjugating E2 enzymes by stabilizing the CHN‐1 U‐box dimer. However, HSP70/HSP‐1 chaperone outcompetes UFD‐2 for CHN‐1 binding, thereby promoting a shift to the autoinhibited CHN‐1 state by acting on a conserved residue in its U‐box domain. The interaction with UFD‐2 enables CHN‐1 to efficiently ubiquitylate and regulate S‐adenosylhomocysteinase (AHCY‐1), a key enzyme in the S‐adenosylmethionine (SAM) regeneration cycle, which is essential for SAM‐dependent methylation. Our results define the molecular mechanism underlying the synergistic cooperation of CHN‐1 and UFD‐2 in substrate ubiquitylation. Synopsis: The regulation of CHIP quality control ubiquitin ligase activity and its substrate selectivity is largely unclear. Here, biochemical, computational, proteomic, and lipidomic data unravel the regulatory mechanism of CHIP processivity, and reveal its role in lipid metabolism. The E3 ligase UFD‐2 stimulates the ubiquitylation activity of CHIP/CHN‐1UFD‐2 binding promotes dimerization of the CHIP/CHN‐1 U‐box domains and E2 enzyme discharging capacityHSP70/HSP‐1, by latching the U‐box and TPR domains, stabilizes the autoinhibitory state of CHIP/CHN‐1, thus limiting its interactions with E2s and UFD‐2Assembly with UFD‐2 enables CHIP/CHN‐1 to regulate lipid metabolism via S‐adenosylhomocysteinase (AHCY‐1) ubiquitylation [ABSTRACT FROM AUTHOR]