Human XTP3-B Forms an Endoplasmic Reticulum Quality Control Scaffold with the HRD1-SEL1L Ubiquitin Ligase Complex and BiP^*

Abstract

The recognition of terminally misfolded proteins in the endoplasmic reticulum (ER) and the extraction of these proteins to the cytoplasm for proteasomal degradation are determined by a quality control mechanism in the ER. In yeast, Yos9p, an ER lectin containing a mannose 6-phosphate receptor homology (MRH) domain, enhances ER-associated degradation (ERAD) of glycoproteins. We show here that human XTP3-B (hXTP3-B), an ER lectin containing two MRH domains, has two transcriptional variants, and both isoforms retard ERAD of the human α₁-antitrypsin variant null Hong Kong (NHK), a terminally misfolded glycoprotein. The hXTP3-B long isoform strongly inhibited ERAD of NHK-QQQ, which lacks all of the N-glycosylation sites of NHK, but the short transcriptional variant of hXTP3-B had almost no effect. Examination of complex formation by immunoprecipitation and by fractionation using sucrose density gradient centrifugation revealed that the hXTP3-B long isoform associates with the HRD1-SEL1L membrane-anchored ubiquitin ligase complex and BiP, forming a 27 S ER quality control scaffold complex. The hXTP3-B short isoform, however, is excluded from scaffold formation. Another MRH domain-containing ER lectin, hOS-9, is incorporated into this large complex, but gp78, another mammalian homolog of the yeast ubiquitin ligase Hrd1p, is not. Based on these results, we propose that this large ER quality control scaffold complex, containing ER lectins, a chaperone, and a ubiquitin ligase, provides a platform for the recognition and sorting of misfolded glycoproteins as well as nonglycosylated proteins prior to retrotranslocation into the cytoplasm for degradation.