A Novel Route for F-box Protein-mediated Ubiquitination Links CHIP to Glycoprotein Quality Control

Abstract

In SCF (Skp1/Cullin/F-box protein) ubiquitin ligases, substrate specificity is conferred by a diverse array of F-box proteins. Only in fully assembled SCF complexes, it is believed, can substrates bound to F-box proteins become ubiquitinated. Here we show that Fbx2, a brain-enriched F-box protein implicated in the ubiquitination of glycoproteins discarded from the endoplasmic reticulum, binds the co-chaperone/ubiquitin ligase CHIP (C terminus of Hsc-70-interacting protein) through a unique N-terminal PEST domain in Fbx2. CHIP facilitates the ubiquitination and degradation of Fbx2-bound glycoproteins, including unassembled NMDA receptor subunits. These findings indicate that CHIP acts with Fbx2 in a novel ubiquitination pathway that links CHIP to glycoprotein quality control in neurons. In addition, they expand the repertoire of pathways by which F-box proteins can regulate ubiquitination and suggest a new role for PEST domains as a protein interaction motif.

Ubc13 and COOH Terminus of Hsp70-interacting Protein (CHIP) Are Required for Growth Hormone Receptor Endocytosis

“…On the cooperativity of CHIP with other E3s, two action modes have been proposed: 1) in neurons, CHIP acts with Fbx2 in glycoprotein quality control, where an NH 2 -terminal PEST sequence interacts with the TPR domain of CHIP to facilitate the F-box associated domain (FBA)-high mannose glycoprotein interaction (22), and 2) in the Notch-induced degradation of Tal1/SCL, where both CHIP and Skp2 can bind Tal1 (23). The latter case bears some similarity with the current study in that the substrate (GHR) binds two E3s, independently.…”

“…Given the role of CHIP in ERAD, it is important to investigate a possible role in GHR synthesis and maturation (22,34,35). Previously, we showed that the GHR is efficiently folded in the endoplasmic reticulum (36).…”

Heat Shock Protein Cognate 70-4 and an E3 Ubiquitin Ligase, CHIP, Mediate Plastid-Destined Precursor Degradation through the Ubiquitin-26S Proteasome System inArabidopsis

“…RbcS [02]:GFP and T7:Hsc70-4 were transformed into protoplasts together with T7:CHIP or T7:TPR, and Hsc70-4-mediated protein degradation was examined in the protoplasts. TPR, a deletion mutant that contains only the N-terminal TPR domain of CHIP (which interacts with Hsp70; Nelson et al, 2006), was included as a dominant-negative mutant ). Coexpression of both T7:CHIP and T7:Hsc70-4 slightly further increased the extent of Hsc70-4-induced reporter protein degradation compared with the expression of T7:Hsc70-4 alone ( Figure 8C, compare lane 5 …”

FBG1 is a promiscuous ubiquitin ligase that sequesters APC2 and causes S-phase arrest

“…[19][20][21] While Skp2 and many other ubiquitin ligases recognize phosphorylated substrates, the FBG family, comprised of FBG1-5, is unique in that it recognizes glycosylated proteins. [22][23][24][25][26][27] In previous studies, we found that overexpressing FBG1 in CHO cells resulted in growth arrest. 28 We and others also previously showed that FBG1 had a relatively low affinity for the SCF scaffold protein Cul1; in fact, rather than forming the canonical SCF complex, FBG1 primarily formed a dimer with Skp1.…”

“…In our earlier work we noted that FBG1 had a low affinity for the SCF scaffold protein, Cul1. 22,23,33 In fact, rather than forming the canonical SCF complex, it primarily formed a dimer with Skp1. Since Cul7 is the only other known cullin that binds F-box proteins ( Fig.…”