Apoprotein B100 (apoB) is a secretory protein that appears to be constitutively translated but inefficiently translocated into the lumen of the endoplasmic reticulum. Using several experimental approaches, we found that apoB is bound to the cytosolic chaperone protein, heat shock protein 72/73 (commonly referred to as Hsp70). Similar to other chaperone-protein interactions, this binding was transient and ATP-sensitive. The binding of apoB to Hsp70 in HepG2 cells was decreased by treatment with oleic acid, which increases both translocation and secretion of apoB, and was increased by N-acetyl-leucyl-leucyl-norleucinal, a protease inhibitor which efficiently protects apoB from cellular degradation without affecting translocation. The N-terminal 16% of apoB, which is efficiently translocated into the endoplasmic reticulum lumen in stably transfected Chinese hamster ovary (CHO) cells, showed minimal, if any, binding to Hsp70. The N-terminal 50% of apoB, which is very poorly translocated in CHO cells, was found to bind significantly to Hsp70. These results suggest that domains of nascent apoB localized on the C-terminal regions of the molecule are transiently exposed to the cytosol during translation and/or translocation, and that Hsp70 functions as a molecular chaperone to maintain apoB in a translocational competent conformation until translocation is completed.

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