AbstractCholesterol side-chain cleavage is sensitive to antibiotic inhibitors of protein synthesis, suggesting that a labile protein may play a regulatory role in this process. We have previously characterized such a factor - steroidogenesis activator polypeptide (SAP). Given the low molecular weight of SAP (Mr3215), a SAP precursor has been sought. Using immunoblotting techniques with two polyclonal antisera directed against portions of the SAP sequence, a single protein of apparentMr82,000 (p82) can be detected in rat adrenocortical tissue. Our data suggest that adrenal p82 is most likely the widely-distributed minor heat shock protein, glucose regulated protein 78 (GRP78). The two proteins share biochemical attributes, includingpI (5.2) and ATP affinity, and the reported amino acid sequences for SAP and for the carboxyl-terminal end of GRP78 are nearly identical. We propose that SAP is cleaved from GRP78 - or a cognate protein - and that this proteolysis is regulated in a manner characteristic of steroidogenic tissues.