Dissection of the translocation and chaperoning functions of yeast BiP/Kar2p in vivo

Abstract

ABSTRACT We used the rat nerve growth factor receptor ectodomain (NGFRe) and Escherichia coli β-lactamase to dissect the functions of Saccharomyces cerevisiae BiP/Kar2p in vivo. Both were fused to the Hsp150Δ-polypeptide, which promotes proper folding of heterologous proteins which otherwise are misfolded in the yeast ER. Hsp150Δ-NGFRe and Hsp150Δ-β-lactamase acquired disulfides and were properly folded and secreted to the culture medium. When disulfide formation was prevented by incubating cells with dithiothreitol (DTT), Hsp150Δ-NGFRe remained in the endoplasmic reticulum (ER). The occupancy of an otherwise partially used N-glycosylation site of reduced NGFRe was complete suggesting that, normally, folding and disulfide formation occurred as rapidly as N-glycosylation. Removal of DTT resulted in remarkably rapid disulfide formation and secretion, suggesting only mild conformational distortion of reduced NGFRe. In contrast, reduced Hsp150Δ-β-lactamase was severely misfolded and attained a secretion competent conformation more slowly after reoxidation. When kar2-159 cells were incubated at permissive temperature 24°C with DTT, the reporter proteins were retained in the ER. After shift of the cells to 34°C to inactivate BiP/Kar2p irreversibly, and subsequent removal of DTT, most pre-accumulated Hsp150Δ-NGFRe was rapidly secreted, whereas Hsp150Δ-β-lactamase was secretion incompetent. Thus, Hsp150Δ-NGFRe did not require BiP/Kar2p for conformational maturation, though translocation was dependent on BiP/Kar2p. Apparently proteins differ in their post-translocational requirements for BiP/Kar2p, indicating that translocation and chaperoning are distinct functions.