Multiple Molecular Chaperones Interact with Apolipoprotein B during Its Maturation

Abstract

The present study was undertaken to identify and characterize molecular chaperones that assist in the folding of apolipoprotein (apo) B, a secretory protein that requires assembly with lipids (lipidation) for its secretion. Both HepG2 cells, normally secreting full-length apoB (apoB-100), and C127 cells transfected to secrete truncated forms of apoB, apoB-41, apoB-29, and apoB-17, respectively, were employed. C127 cells were used to determine whether chaperone binding is dependent on apoB lipidation as they secrete both unlipidated and lipidated apoB forms despite their lack of microsomal triglyceride transfer protein (MTP), which mediates lipidation of apoB in HepG2 cells. The endoplasmic reticulum (ER)-resident molecular chaperones GRP94, calreticulin, and ERp72 were co-immunoprecipitated with apoB-100 from HepG2 cell lysates following cross-linking of proteins in living cells. The same chaperones including BiP/GRP78 were also associated with all truncated forms of apoB. Sequential immunoprecipitation with antibodies to MTP and apoB revealed the presence of ternary complexes containing apoB-100, MTP, and ERp72. However, MTP is not obligatory for the binding of ERp72 as it was associated with all truncated forms of apoB in C127 cells that lack MTP. The interactions between apoB-100 and ERp72 or GRP94 persisted for at least 2 h following a 30-min pulse. Thus, BiP/GRP78, calreticulin, ERp72, and GRP94 may participate in critical steps in the folding of apoB before any substantial lipidation occurs. ERp72 and GRP94 may also mediate the folding of more advanced folding intermediates and/or target the misfolded underlipidated pool of apoB for degradation. The present study was undertaken to identify and characterize molecular chaperones that assist in the folding of apolipoprotein (apo) B, a secretory protein that requires assembly with lipids (lipidation) for its secretion. Both HepG2 cells, normally secreting full-length apoB (apoB-100), and C127 cells transfected to secrete truncated forms of apoB, apoB-41, apoB-29, and apoB-17, respectively, were employed. C127 cells were used to determine whether chaperone binding is dependent on apoB lipidation as they secrete both unlipidated and lipidated apoB forms despite their lack of microsomal triglyceride transfer protein (MTP), which mediates lipidation of apoB in HepG2 cells. The endoplasmic reticulum (ER)-resident molecular chaperones GRP94, calreticulin, and ERp72 were co-immunoprecipitated with apoB-100 from HepG2 cell lysates following cross-linking of proteins in living cells. The same chaperones including BiP/GRP78 were also associated with all truncated forms of apoB. Sequential immunoprecipitation with antibodies to MTP and apoB revealed the presence of ternary complexes containing apoB-100, MTP, and ERp72. However, MTP is not obligatory for the binding of ERp72 as it was associated with all truncated forms of apoB in C127 cells that lack MTP. The interactions between apoB-100 and ERp72 or GRP94 persisted for at least 2 h following a 30-min pulse. Thus, BiP/GRP78, calreticulin, ERp72, and GRP94 may participate in critical steps in the folding of apoB before any substantial lipidation occurs. ERp72 and GRP94 may also mediate the folding of more advanced folding intermediates and/or target the misfolded underlipidated pool of apoB for degradation. Apolipoprotein B (apoB) 1The abbreviations used are: apoBapolipoprotein BMTPmicrosomal triglyceride transfer proteinMTPLMTP large subunitBiPimmunoglobulin heavy chain-binding proteinPDIprotein-disulfide isomeraseERendoplasmic reticulumCHAPS3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfanateTAGtriacylglycerolDMEMDulbecco's modified Eagle's mediumPBSphosphate-buffered salineAEBSF4-[2-aminoethyl]benzylsulfonyl fluorideALLNN-acetyl-leucinyl-leucinyl-norleucinalDSPdithiobis(succinimidyl propionate)DPBSDulbecco's phosphate-buffered saline. is an atypical secretory protein. Its folding into a mature, secretion-competent form necessitates assembly with triacylglycerols (TAG) and phospholipids to produce TAG-rich lipoproteins such as very low density lipoproteins and chylomicrons (1Kane J.P. Havel R.J. Scriver C.R. Beaudet A.L. Sly W.S. Valle D. in The Metabolic Basis of Inherited Disease. 6th Ed. McGraw-Hill, New York1983: 1139-1164Google Scholar). A major fraction of nascent apoB is degraded intracellularly (2Borchardt R.A. Davis R.A. J. Biol Chem. 1987; 262: 16394-16402Abstract Full Text PDF PubMed Google Scholar) when lipid availability is limited or when lipidation of apoB is inhibited (reviewed in Refs. 3Ginsberg H.N. Clin. Exp. Pharmacol. Physiol. 1997; 24: A29-A32Crossref PubMed Scopus (49) Google Scholar and 4Yao Z. Tran K. McLeod R.S. J. Lipid Res. 1997; 38: 1937-1953Abstract Full Text PDF PubMed Google Scholar). It was proposed that distinct amphipathic elements in the secondary structure of apoB may initiate the binding of lipids to apoB co-translationally (5Small D.M. Atkinson D. Circulation. 1997; 96: 3PubMed Google Scholar). Further, growing evidence suggests that the process of apoB maturation in the cell is mediated by molecular chaperones (3Ginsberg H.N. Clin. Exp. Pharmacol. Physiol. 1997; 24: A29-A32Crossref PubMed Scopus (49) Google Scholar, 4Yao Z. Tran K. McLeod R.S. J. Lipid Res. 1997; 38: 1937-1953Abstract Full Text PDF PubMed Google Scholar), which by binding nascent polypeptides prevent their aggregation and direct their folding into their mature, native form (6Hartl F.U. Nature. 1996; 381: 571-580Crossref PubMed Scopus (3130) Google Scholar, 7Gething M.J. Sambrook J. Nature. 1992; 355: 33-45Crossref PubMed Scopus (3607) Google Scholar). Indeed, the recruitment of lipids to the nascent lipoprotein is significantly aided by microsomal triglyceride transfer protein (MTP) localized to the lumen of the endoplasmic reticulum (ER) (reviewed in Ref. 8Wetterau J.R. Lin M.C. Jamil H. Biochim. Biophys. Acta. 1997; 1345: 136-150Crossref PubMed Scopus (286) Google Scholar). MTP is a heterodimer composed of a 97-kDa subunit (MTPL) and protein-disulfide isomerase (PDI). The latter belongs to the family of the abundant thioredoxin-like proteins in the ER lumen, which catalyze the formation and rearrangement of disulfide bonds. PDI also functions as a molecular chaperone, and this function is independent of its enzymatic activity (reviewed in Ref. 9Luz J.M. Lennarz W.J. EXS. 1996; 77: 97-117PubMed Google Scholar). In human hepatoma-derived HepG2 cells, besides MTP (10Wu X. Zhou M. Huang L.S. Wetterau J. Ginsberg H.N. J. Biol. Chem. 1996; 271: 10277-10281Abstract Full Text Full Text PDF PubMed Scopus (124) Google Scholar), three other ER-resident molecular chaperones, calnexin, ER60/ER57, and BiP/GRP78, have been demonstrated to interact with apoB (11Ou W-J. Cameron P.H. Thomas D.Y. Bergeron J.M. Nature. 1993; 364: 771-776Crossref PubMed Scopus (488) Google Scholar, 12Adeli K. Marci J. Mohammadi A. Kito M. Urade R. Cavallo D. J. Biol. Chem. 1997; 272: 22489-22494Abstract Full Text Full Text PDF PubMed Scopus (68) Google Scholar). Calnexin, an integral membrane protein (reviewed in Ref.13), and ER60, a thioredoxin-like protein (14Hirano N. Shibasaki F. Sakai R. Tanaka T. Nishida J. Yazaki Y. Takenawa T. Hirai H. Eur. J. Biochem. 1995; 234: 336-342Crossref PubMed Scopus (122) Google Scholar) and a protease (15Urade R. Nasu M. Moriyama T. Wada K. Kito M. J. Biol. Chem. 1992; 267: 15152-15159Abstract Full Text PDF PubMed Google Scholar), exhibit specificity for nascent unfolded polypeptides bearingN-linked monoglucosylated oligosaccharides (13Hammond C. Helenius A. Curr. Biol. 1993; 3: 884-886Abstract Full Text PDF PubMed Scopus (46) Google Scholar, 16Oliver J.D. van der Wal F.J. Bulleid N.J. High S. Science. 1997; 275: 86-88Crossref PubMed Scopus (347) Google Scholar, 17Elliott J.G. Oliver J.D. High S. J. Biol. Chem. 1997; 272: 13849-13855Abstract Full Text Full Text PDF PubMed Scopus (119) Google Scholar). BiP, initially identified as immunoglobulin binding protein, is the ER homologue of the cytosolic Hsp70 chaperone, and is thought to interact with unshielded hydrophobic domains of unfolded proteins (18Knarr G. Gething M.-J. Modrow S. Buchner J. J. Biol. Chem. 1995; 270: 27589-27594Abstract Full Text Full Text PDF PubMed Scopus (98) Google Scholar). BiP has been suggested to facilitate the movement of nascent polypeptides through the translocon into the ER lumen (19Lyman S.K. R. 1997; Full Text Full Text PDF PubMed Scopus Google Scholar) as as the of misfolded proteins from the ER into the for S. J. T. Nature. 1997; PubMed Scopus Google Scholar). In a study in HepG2 cells, the binding of the cytosolic chaperone Hsp70 to apoB-100 was to Zhou M. D.M. X. S. H. A.L. Ginsberg H.N. J. Biol. Chem. 1997; 272: Full Text Full Text PDF PubMed Scopus Google Scholar). apolipoprotein B microsomal triglyceride transfer protein MTP large subunit immunoglobulin heavy chain-binding protein protein-disulfide isomerase endoplasmic reticulum modified Eagle's phosphate-buffered phosphate-buffered saline. In this whether other abundant ER-resident proteins to function as molecular chaperones participate in the folding of apoB, and whether the interactions dependent on the of apoB identify chaperones that interact with apoB, used human HepG2 cells normally secreting the full-length apoB-100, as as transfected C127 cells from secreting truncated forms of apoB to the and of apoB-100, apoB-41, apoB-29, and apoB-17, The transfected C127 cells were as a to study the of the interactions on the lipidation of apoB for cells of apoB and MTP, and is by cells on density H. A. D.M. S. A. 1995; PubMed Scopus Google Scholar), K. M. and D. M. and H. M. C. D.M. S. A. PubMed Scopus Google Scholar) or lipid to that three other ER-resident molecular chaperones interact with calreticulin, ERp72, and a major protein, has been to function as a molecular chaperone which for (reviewed in Ref. M. 1997; Full Text Full Text PDF PubMed Scopus Google Scholar). However, the of ERp72, a of the family R. M. M. J. Biol. Chem. Full Text PDF PubMed Google Scholar), and GRP94, a protein to the cytosolic R.A. M. J. Biol. Chem. 1987; 262: Full Text PDF PubMed Google Scholar), in protein folding Both proteins exhibit binding to proteins in S.K. A.L. S. J. Biol. Chem. Full Text PDF PubMed Google Scholar). have also been to intracellularly with nascent polypeptides such as G. S.K. J. Biol. Chem. Full Text PDF PubMed Google Scholar), G. S.K. J. Biol. Chem. 1997; 272: Full Text Full Text PDF PubMed Scopus Google Scholar), major M. J. Exp. 1992; PubMed Scopus (124) Google Scholar), and human subunit J. Biol. Chem. 1996; 271: Full Text Full Text PDF PubMed Scopus Google Scholar). Further, their K. A.L. S.K. J. Biol. Chem. 1997; 272: Full Text Full Text PDF PubMed Scopus Google Scholar, M. G. Exp. PubMed Scopus Google Scholar). that GRP94 and ERp72 interact with both and more advanced folding intermediates of apoB and mediate its folding into a secretion-competent chaperones were to interact with apoB whether it is lipidated or modified Eagle's and were from protein with activity and for were from antibodies to human apoB were from antibodies to ERp72 was by antibodies to GRP94, antibodies to BiP, antibodies to and antibodies to were from antibodies to the large subunit of MTP were by Wetterau and were from was from was from was from membrane was from HepG2 cells and C127 cells transfected with truncated forms of apoB were to as H. A. D.M. S. A. 1995; PubMed Scopus Google Scholar, H. M. C. D.M. S. A. PubMed Scopus Google Scholar). to the cells were in phosphate-buffered and in and for were in and with in for in containing and 2 and and in the same for 2 h the of or cells were by in and in with protease and and a 30-min as cells were by and in were on by of which in was with and or or and cross-linking was were cross-linking cells were for on in containing and from a in J. Biol. PubMed Scopus Google Scholar). cross-linking was for by the lysates to were with protease as in of proteins in living cells, cells were for on with in to and for were and in with protease as to cell lysates were by with protein for at were at with antibodies or in were to binding to protein J. Biol. PubMed Scopus Google Scholar) and with protein for h at to The were in the following same with and immunoprecipitation was were to a for in in with in with a and on protein were with and in and at for in Nature. PubMed Scopus Google Scholar) containing and and by modified from Nature. 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Nature. 1992; 355: 33-45Crossref PubMed Scopus (3607) Google Scholar, W.J. A.L. Nature. 1993; PubMed Scopus (347) Google Scholar) in the presence of Thus, the of and/or of cells may prevent the of chaperones and their G. S.K. J. Biol. Chem. Full Text PDF PubMed Google M. X. Huang Ginsberg H.N. J. Biol. Chem. 1995; 270: Full Text Full Text PDF PubMed Scopus Google Scholar). However, for chaperones that or when interactions cross-linking of proteins to the of complexes G. S.K. J. Biol. Chem. Full Text PDF PubMed Google Scholar, J. Biol. PubMed Scopus Google Scholar, J. J. Y. Nature. PubMed Scopus Google Scholar). is folding of apoB, it was to that to as chaperones as that HepG2 cells were for and to by immunoprecipitation with antibodies to apoB The following were in and and to 2 an and or and and The cross-linking was in distinct the of and and to in cross-linking in living cells, to for in of chaperones with apoB before of cells that complexes that the ER-resident GRP94, the human homologue of ERp72 and calreticulin, and the cytosolic chaperone were co-immunoprecipitated with apoB The that were in or with of the chaperones The of chaperones that co-immunoprecipitated with apoB despite a in the of immunoprecipitation of apoB. The of GRP94 and ERp72 co-immunoprecipitated with apoB-100 were in cross-linking of proteins The to the of a fraction of chaperones before the cross-linking to chaperones were to apoB in living cells, that they interact with apoB in G. S.K. J. Biol. Chem. Full Text PDF PubMed Google Scholar, G. S.K. J. Biol. Chem. 1997; 272: Full Text Full Text PDF PubMed Scopus Google Scholar) also that the interactions between GRP94 or ERp72 and and in were by and cross-linking of proteins in their of GRP94 associated with immunoglobulin in cells also cross-linking of proteins J. J. Y. Nature. PubMed Scopus Google Scholar). GRP94 was to for its from apoB-100, as of GRP94 were co-immunoprecipitated with apoB a of the by the of GRP94 were co-immunoprecipitated with apoB from the lysates or from the lysates with and Thus, the lysates of of GRP94 In to J. J. Y. Nature. PubMed Scopus Google Scholar) that a of the of in cells to the of GRP94 from immunoglobulin this both study and that of that GRP94 may an protein. However, the binding of to GRP94 also in other Z. J. 1993; PubMed Scopus Google Scholar, A. J. Biol. Chem. 1992; 267: Full Text PDF PubMed Google Scholar, Y. T. J. Biol. Chem. 1995; 270: Full Text Full Text PDF PubMed Scopus Google Scholar) was by and J. Biol. Chem. 1997; 272: Full Text Full Text PDF PubMed Scopus Google Scholar), suggested that the binding and of is not an of However, were by of GRP94 Z. J. 1993; PubMed Scopus Google Scholar, Y. T. J. Biol. Chem. 1995; 270: Full Text Full Text PDF PubMed Scopus Google Scholar) and from the structure of its cytosolic homologue C. R. 1997; Full Text Full Text PDF PubMed Scopus Google Scholar). In of this it whether the of GRP94 from apoB binding of to GRP94 or to other proteins that may present in the is with the that chaperones in the ER form large with their polypeptides Helenius A. J. Biol. 1997; PubMed Scopus Google Scholar). Further, of the by proteins has been for cytosolic chaperones S. T. H. Buchner J. Science. 1996; PubMed Scopus Google Scholar, J. Biol. Chem. 1997; 272: Full Text Full Text PDF PubMed Scopus Google Scholar). Thus, the following of on the between GRP94 and apoB-100 or immunoglobulin may to in the of chaperones present in and GRP94 and ERp72, the chaperone and the cytosolic chaperone form of not cross-linking they were other was to in the presence of the and that the of from apoB was when in the lysates were In the homologue of calreticulin, calnexin, to for its with a of A. J. Biol. Chem. Full Text PDF PubMed Google Scholar). on the other was to of complexes in A. N. A. A.L. A. J. Biol. Chem. 1997; 272: Full Text Full Text PDF PubMed Scopus Google Scholar). for the between and apoB-100 in HepG2 cells was in a nascent apoB was by antibodies to following in not that with apoB in and that the of to apoB on the was a The cytosolic was when the of in the was by The of Hsp70 from its has been demonstrated in other W.J. A.L. Nature. 1993; PubMed Scopus (347) Google Scholar). The of with apoB was in a in which a of cells were to in and with antibodies to apoB. was by and H. Zhou M. D.M. X. S. H. A.L. Ginsberg H.N. J. Biol. Chem. 1997; 272: Full Text Full Text PDF PubMed Scopus Google Scholar) have that of HepG2 cells with form of to of However, in their antibodies both forms of Hsp70 were it to whether or both forms Zhou M. D.M. X. S. H. A.L. Ginsberg H.N. J. Biol. Chem. 1997; 272: Full Text Full Text PDF PubMed Scopus Google Scholar). evidence that with apoB-100 its may direct the misfolded pool of apoB for degradation. is with a in the of protein in A. N. A. A.L. A. J. Biol. Chem. 1997; 272: Full Text Full Text PDF PubMed Scopus Google Scholar). In the in the chaperones identified by were associated with both nascent apoB the 30-min and the pool that was present in the cells at the of the pulse. A fraction of this pool for degradation. It was to determine fraction of the nascent apoB which folding was associated with chaperones, and chaperones with apoB. whether antibodies GRP94 or ERp72 nascent apoB. that the antibodies to GRP94 and ERp72 not with apoB the following HepG2 cells were for and by to in with or to all associated proteins J. Biol. Chem. 1996; 271: Full Text Full Text PDF PubMed Scopus Google Scholar). nascent apoB-100 not HepG2 cells were for and for 2 cross-linking of proteins in cells were and to immunoprecipitation with antibodies ERp72, GRP94, or apoB. 2 that apoB-100 was by antibodies to GRP94 or ERp72 both the and a Thus, chaperones interact with apoB-100 its and interactions persisted for at least 2 h into the A and The of the nascent apoB associated with chaperone was at and 2 h least and of the pool of apoB was by antibodies to ERp72 and GRP94, However, in the same the of apoB to not were also by GRP94 and ERp72 may a in both and more advanced in the folding of apoB and its assembly with have been on ERp72, J. J. Y. Nature. PubMed Scopus Google Scholar) demonstrated that GRP94 has a more advanced folding intermediates of immunoglobulin BiP and were to to very folding intermediates and their to the of folding J. Biol. 1995; PubMed Scopus Google J. J. Y. Nature. PubMed Scopus Google Scholar). However, folding into a native form was not both BiP and to their (11Ou W-J. Cameron P.H. Thomas D.Y. Bergeron J.M. Nature. 1993; 364: 771-776Crossref PubMed Scopus (488) Google Scholar, A. J. Biol. Chem. 1996; 271: Full Text Full Text PDF PubMed Scopus Google Scholar) and the misfolded polypeptides were for A. J. Biol. Chem. 1996; 271: Full Text Full Text PDF PubMed Scopus Google Scholar). The of GRP94 and ERp72 with apoB-100 suggests that chaperones may target the misfolded apoB for degradation. a for GRP94 in protein has not been its cytosolic homologue was suggested to facilitate protein C. S. N. F.U. 1996; PubMed Scopus Google Scholar). ERp72, on the other has been to a activity that is to the R. Y. Kito M. J. Biol. Chem. 1993; Full Text PDF PubMed Google Scholar). of apoB-100 very it is that ERp72 may in the of a of misfolded apoB. The folding of apoB in the ER into a secretion-competent form is it assembly with The ER-resident protein MTP an in the lipidation of apoB it has been to catalyze the transfer of and phospholipids in and in the or of MTP activity in a substantial or of apoB J.R. Lin M.C. Jamil H. Biochim. Biophys. Acta. 1997; 1345: 136-150Crossref PubMed Scopus (286) Google Scholar). of truncated forms of apoB to or in with J.R. Lin M.C. Jamil H. Biochim. Biophys. Acta. 1997; 1345: 136-150Crossref PubMed Scopus (286) Google Scholar). Thus, it was to determine whether GRP94 and ERp72 interact with apoB in with MTP. that HepG2 cells were for and proteins were in cross-linking was it in lysates were to immunoprecipitation with antibodies A that both GRP94 and ERp72 were co-immunoprecipitated with The antibodies to not chaperone from C127 cell lysates that lack that in HepG2 cells MTP with both chaperones in immunoprecipitation of cell lysates following a 30-min was with antibodies to and with antibodies to apoB by B that ERp72 was in the presence of ternary complexes containing nascent apoB-100, ERp72, and MTP. that the function of ERp72 as disulfide isomerase may with the of lipidation of folding intermediates of apoB. ERp72, GRP94 was not in complexes containing both apoB and MTP the of whether the binding of chaperones to apoB is by the of the apoB truncated forms of apoB by transfected C127 cells. the of MTP in cells, they secrete on density TAG-rich in C127 and apoB-100 in HepG2 cells in that their of by lipid availability (3Ginsberg H.N. Clin. Exp. Pharmacol. Physiol. 1997; 24: A29-A32Crossref PubMed Scopus (49) Google Scholar, H. A. D.M. S. A. 1995; PubMed Scopus Google Scholar). of the of is the truncated form of apoB, apoB-29, is with of which is phospholipids and The apoB-17, is any of lipid H. M. C. D.M. S. A. PubMed Scopus Google Scholar). were on the in HepG2 cells. were to immunoprecipitation with antibodies to apoB. The were to the chaperones identified in HepG2 cells as as BiP, which has been to interact with apoB-100 in HepG2 cells K. Marci J. Mohammadi A. Kito M. Urade R. Cavallo D. J. Biol. Chem. 1997; 272: 22489-22494Abstract Full Text Full Text PDF PubMed Scopus (68) Google Scholar). The chaperones GRP94, ERp72, and calreticulin, as as BiP, were to interact with all three truncated forms of apoB and GRP94, ERp72, and BiP were in to all three forms was to and Thus, that chaperones interact with truncated forms of apoB in and their with apoB is independent of the lipidation of apoB. chaperones may in steps of apoB folding that before lipidation chaperones with in C127 cells. were were with antibodies to apoB or in the complexes were by with antibodies The following were and in 2 and in presence of and and in the presence and in cross-linking with to in chaperones with in C127 cells. were were with antibodies to apoB or in the and complexes were by with antibodies The following were in in the presence and in cross-linking with to in all chaperones interact with all forms of apoB in cell HepG2 and the for for their to dependent on the of apoB, and the cell was for the of GRP94 from apoB-100 in HepG2 cells not from the truncated forms of apoB A and on the other for its from not from or apoB-100 is not an protein, it is that the from was to other present in The to the that the chaperones identified in this study with apoB forms as a of complexes containing other chaperones or proteins that may an in the folding of apoB into a secretion-competent in HepG2 cells, ERp72 is in both B, and 2 in the folding of apoB. demonstrated that the folding intermediates of apoB with both ERp72 and MTP that the proteins may function in C127 cells that ERp72 intracellularly with all truncated forms of apoB and it was not whether ERp72 to truncated forms in their or it into that ERp72 interact with folding intermediates of apoB in the of MTP, C127 cells were for and in the presence of the cross-linking was following with and with antibodies that MTP is not for the of ERp72 with nascent apoB. ERp72 is a of the family of and it may in the disulfide formation in apoB. the in apoB-100, the to the of apoB M. S. A. PubMed Scopus Google Scholar). by and in HepG2 cells J. Lipid Res. 1996; Full Text PDF PubMed Google Scholar) and D. M. and H. in in C127 cells H. D.M. Circulation. 1996; Scholar), suggested that the disulfide formation in the of apoB is critical for its folding into a secretion-competent were by that of the of apoB J. Biol. Chem. 1997; 272: Full Text Full Text PDF PubMed Scopus Google Scholar, K. J. J. Y. McLeod R. K. Z. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar) in the of apoB to with lipids and its in the cell as a misfolded protein. It is that of the family such as ERp72, or apoB very its to catalyze the of to the binding of MTP, which in may initiate the recruitment of It is at present whether of chaperones is in the process and whether is a of or more thioredoxin-like Both ERp72 and K. Marci J. Mohammadi A. Kito M. Urade R. Cavallo D. J. Biol. Chem. 1997; 272: 22489-22494Abstract Full Text Full Text PDF PubMed Scopus (68) Google Scholar) were to interact with apoB. It to whether ERp72 and in in disulfide or whether their chaperone their isomerase activity a more in apoB In the major of the ER chaperone GRP94, ERp72, BiP, and calreticulin, were to interact with apoB-100 in HepG2 cells and its truncated forms in C127 cells. Thus, the truncated forms were to an to determine whether the chaperones between lipidation of apoB. that chaperones apoB before any substantial lipidation that they may in the critical folding steps to assembly with that of steps may the binding of ERp72 to the of apoB to mediate disulfide formation to for MTP to and assist the assembly with The in the of the interactions between apoB forms and the chaperones that the chaperones with apoB as a of complexes containing a of chaperones and the interactions of ERp72 and GRP94 with apoB-100 for at least 2 it is that chaperones also in in the folding of apoB, of the misfolded apoB. M. for this and for very from for ERp72 Wetterau from for MTP and from the of at for the C127 cells the truncated forms of for to this and for the cells.