Processing and secretion of the Yarrowia lipolytica RNase

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Abstract
Secretion of the extracellular RNase from the yeast Yarrowia lipolytica was studied in pulse-chase and immunoprecipitation experiments. A polypeptide of 45,000 daltons was immunoprecipitated from [35S]methionine-labeled cell extracts and supernatant medium by rabbit anti-RNase antiserum. The RNase was secreted rapidly; the time between synthesis and appearance in the extracellular medium was about 5 min. In pulse-chase experiments, about 50% of the RNase was still cell associated 30 min after labeling. A polypeptide of 73,000 daltons whose immunoprecipitation was blocked by an excess of purified RNase was also detected. It broke down to a polypeptide with the same mobility and same peptide map as the mature RNase. Peptide maps of the undegraded 73-kilodalton polypeptide and the intracellular mature RNase contained several peptides of identical mobility. Immunoprecipitates from cells labeled in the presence of tunicamycin contained 66- and 45-kilodalton polypeptides. Endoglycosidase H treatment of the 73-kilodalton polypeptide converted it to a 66-kilodalton form, but did not change the apparent molecular weight of the mature form of the RNase. Labeling kinetics from pulse-chase experiments did not clearly support a precursor-product relationship between the 73-kilodalton polypeptide and the intracellular 45-kilodalton form of the RNase, and other relationships between the two polypeptides are possible.