title: Role of cysteine-protease CGHC motifs of ER-60, a protein disulfide isomerase, in hepatic apolipoprotein B100 degradation.
abstract: Apolipoprotein B100 (apoB), the structural component of very low density lipoproteins (VLDL), is susceptible to misfolding and subsequent degradation by several intracellular pathways. ER-60, which has been implicated in apoB degradation, is a protein disulfide isomerase (PDI) that forms or rearranges disulfide bonds in substrate proteins and also possesses cysteine protease activity. To determine which ER-60 function is important for apoB degradation, adenoviruses encoding wild-type human ER-60 or a mutant form of human ER-60 (C60A, C409A) that lacked cysteine protease activity were overexpressed in HepG2 cells. Overexpression of wild-type ER-60 in HepG2 cells promoted apoB degradation and impaired apoB secretion, but mutant ER-60 overexpression did not. In McArdle RH-7777 cells, VLDL secretion was markedly inhibited following overexpression of wild-type but not mutant ER-60, an effect that could be blocked by oleate treatment. Mutant ER-60 was not trapped on apoB as it was with the control substrate tapasin, suggesting that ER-60's role in apoB degradation is likely unrelated to its protein disulfide isomerase activity. Thus, ER-60 may participate in apoB degradation by acting as a cysteine protease. We postulate that apoB cleavage by ER-60 within the ER lumen could facilitate proteasomal degradation of the C-terminus of translocationally-arrested apoB.
authors.affiliation: Program in Molecular Structure and Function, Research Institute, The Hospital for Sick Children, Toronto, Ontario, Canada.
authors.forename: Angela C
authors.initials: AC
authors.lastname: Rutledge
authors.forename: Wei
authors.initials: W
authors.lastname: Qiu
authors.forename: Rianna
authors.initials: R
authors.lastname: Zhang
authors.forename: Reiko
authors.initials: R
authors.lastname: Urade
authors.forename: Khosrow
authors.initials: K
authors.lastname: Adeli
mesh_terms: D020816:Amino Acid Motifs; D053299:Apolipoprotein B-100 / Q000737:chemistry* / Q000378:metabolism*; D001665:Binding Sites; D003545:Cysteine / Q000737:chemistry* / Q000378:metabolism*; D004789:Enzyme Activation; D056945:Hep G2 Cells; D006801:Humans; D011485:Protein Binding; D019704:Protein Disulfide-Isomerases / Q000737:chemistry* / Q000378:metabolism*; D013329:Structure-Activity Relationship
chemical_list: D053299:Apolipoprotein B-100; D019704:Protein Disulfide-Isomerases; C085868:PDIA3 protein, human; D003545:Cysteine
keywords: ApoB Degradation; Cysteine protease; ER-60; Endoplasmic reticulum; Protein disulfide isomerase