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(Arteriosclerosis, Thrombosis, and Vascular Biology. 2009;29:579.)
© 2009 American Heart Association, Inc.

Cell Biology/Signaling

Mechanisms Targeting Apolipoprotein B100 to Proteasomal Degradation

Evidence That Degradation Is Initiated by BiP Binding at the N Terminus and the Formation of a p97 Complex at the C Terminus

Angela C. Rutledge; Wei Qiu; Rianna Zhang; Rita Kohen-Avramoglu; Nina Nemat-Gorgani; Khosrow Adeli

From Molecular Structure and Function, Research Institute, The Hospital for Sick Children, Toronto, Ontario, Canada (A.C.R., W.Q., R.Z., R.K-A., N.N.-G., K.A.) and the Department of Laboratory Medicine and Pathobiology, University of Toronto, Ontario, Canada (A.C.R.).

Correspondence to Khosrow Adeli, Molecular Structure and Function, Research Institute, The Hospital for Sick Children, Toronto, Ontario, Canada. E-mail khosrow.adeli{at}sickkids.ca

Objectives— In lipid-poor states, the ubiquitin-proteasomal pathway rapidly degrades misfolded apolipoprotein B100 (apoB) cotranslationally, although the mechanism of delivery from the ER to cytosolic proteasomes is poorly understood. Here we demonstrate key roles of BiP, an ER luminal chaperone, and p97, a cytosolic ATPase anchored to the ER membrane, in the targeting of apoB for proteasomal degradation.

Methods and Results— Using coimmunoprecipitations, we observed associations of apoB with BiP, p97, Derlin-1, VIMP, and the E3 ubiquitin ligase Hrd1 in HepG2 cells. BiP and p97 were found to bind apoB cotranslationally. Expression of C-terminal truncated apoB molecules in COS-7 cells showed an N-terminal region outside apoB15 and a C-terminal region found in apoB72 were required for BiP and p97 binding, respectively. Interestingly, overexpression of dominant negative p97 demonstrated that the ATPase activity of p97 was essential for proteasomal degradation of apoB but not for apoB binding. However, p97 activity did not appear to affect the N terminus of apoB, which may be cleaved before degradation.

Conclusions— These data suggest that p97 and BiP play critical roles in the cotranslational delivery of apoB to proteasomes and formation of a degradative complex. Proteasomal degradation appears to selectively target apoB molecules with large C-terminal domains.

Proteasomal targeting mechanisms mediating cotranslational degradation of apolipoprotein B100 (apoB) were investigated. BiP and p97 were found to respectively bind the N and C termini of apoB cotranslationally and initiate the formation of a degradative complex. p97 was essential for proteasomal degradation of a C-terminal portion of apoB.

Key Words: apolipoprotein B • degradation • p97 • BiP • proteasome