E3 ubiquitin ligase that recognizes sugar chains

N-glycosylation of proteins in the endoplasmic reticulum (ER) has a central role in protein quality control(1-3). Here we report that N-glycan serves as a signal for degradation by the Skp1-Cullin1-Fbx2-Roc1 (SCFFbx2) ubiquitin ligase complex. The F-box protein Fbx2 (ref. 4) binds specifically to proteins attached to N-linked high-mannose oligosaccharides and subsequently contributes to ubiquitination of N-glycosylated proteins. Pre-integrin beta1 is a target of Fbx2; these two proteins interact in the cytosol after inhibition of the proteasome. In addition, expression of the mutant Fbx2DeltaF, which lacks the F-box domain that is essential for forming the SCF complex, appreciably blocks degradation of typical substrates of the ER-associated degradation pathway(5,6). Our results indicate that SCFFbx2 ubiquitinates N-glycosylated proteins that are translocated from the ER to the cytosol by the quality control mechanism.