Proteins of the endoplasmic-reticulum-associated degradation pathway: domain detection and function prediction

Sequence database searches, using iterative-profile and Hidden-Markov-model approaches, were used to detect hithertoundetected homologues of proteins that regulate the endoplasmic reticulum (ER)-associated degradation pathway. The translocon-associated subunit Sec63p(Sec = secretory) was shown to contain a domain of unknown function found twice in several Brr2p-like RNA helicases (Brr2 = bad response to refrigeration 2). Additionally, Cue1p (Cue = coupling of ubiquitin conjugation to ER degradation), a yeast protein that recruits the ubiquitin-conjugating (UBC) enzyme Ubc7p to an ER-associated complex, was found to be one of a large family of putative scaffolding-domain-containing proteins that include the autocrine motility factor receptor and fungal Vps9p (Vps = vacuolar protein sorting). Two other yeast translocon-associated molecules, Sec72p and Hrd3p (Hrd = 3-hydroxy-3-methylglutaryl-CoA reductase degradation), were shown to contain multiple tetratricopeptide-repeat-like sequences. From this observation it is suggested that Sec72p associates with a heat-shock protein, Hsp70, in a manner analogous to that known for Hop (Hsp70/Hsp90 organizing protein). Finally, the luminal portion of Ire1p (Ire = high inositol-requiring), thought to convey the sensing function of this transmembrane kinase and endoribonuclease, was shown to contain repeats similar to those in beta -propeller proteins. This finding hints at the mechanism by which Ire1p may sense extended unfolded proteins at the expense of compact folded molecules.