A conserved ubiquitin ligase of the nuclear envelope/endoplasmic reticulumthat functions in both ER-associated and Mat alpha 2 repressor degradation

Substrate discrimination in the ubiquitin-proteasome system is believed to be dictated by specific combinations of ubiquitin-protein ligases (E3s) and ubiquitin-conjugating enzymes (E2s). Here we identify Doa10/Ssm4 as a yeas t E3 that is embedded in the endoplasmic reticulum (ER)/nuclear envelope ye t can target the soluble transcription factor Mat alpha2 Doa10 contains an unusual RING finger, which has ubiquitin-ligase activity in vitro and is es sential in vivo for degradation of alpha2 via its Deg1 degradation signal. Doa10 functions with two E2s, Ubc6 and Ubc7, to ubiquitinate Deg1-bearing s ubstrates, and it is also required for the degradation of at least one ER m embrane protein. interestingly, different short-lived ER proteins show dist inct requirements for Doa10 and another ER-localized E3, Hrd1. Nevertheless , the two E3s overlap in function: A doa10 Delta hrd1 Delta mutant is far m ore sensitive to cadmium relative to either single mutant and displays stro ng constitutive induction of the unfolded protein response; this suggests a role for both E3s in eliminating aberrant E R proteins. The likely human o rtholog of DOA10 is in the cri-du-chat syndrome critical region on chromoso me 5p, suggesting that defective ubiquitin ligation might contribute to thi s common genetic disorder.