We have characterized the association between the binding protein, BiP (also known as GRP 78), and misfolded forms of the influenza virus hemagglutinin precursor, HA0. BiP is a heat-shock-related protein that binds to unassembled immunoglobulin heavy chain and to a variety of misfolded proteins in the lumen of the ER. A small fraction (5-10%) of newly synthesized HA0 in CV-1 cells was found to be misfolded and retained in the ER. When glycosylation was blocked with tunicamycin, all of the HA0 produced was similarly misfolded. The misfolded HA0 was retained as relatively small (9-25-S) complexes associated with BiP. In these complexes the top domains of HA0 were correctly folded judging by their reactivity with monoclonal antibodies, but the polypeptides were cross-linked via anomalous interchain disulfides. The association with BiP was non-covalent and easily broken by warming to 37 degrees C or by adding ATP to the lysate. Pulse-chase experiments showed that HA0's self-association into complexes occurred immediately after synthesis and was followed rapidly by BiP association. The misfolded, BiP-associated HA0 was not transported to the plasma membrane but persisted as complexes in the ER for a long period of time before degradation (t1/2 = 6 h). The results suggested that BiP may be part of a quality control system in the ER and that one of its functions is to detect and retain misfolded proteins.

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