Multiplicity in cellulases of Schizophyllum commune. Derivation partly from heterogeneity in transcription and glycosylation.

Abstract

The white-rot fungus, Schizophyllum commune, secretes a member of each of three classes of cellulases: a beta-glucosidase, an exoglucanase, and an endoglucanase. Antibodies were developed to members of each of these three enzyme classes. Secretion of these cellulases is induced when a mycelium is transferred from a glucose to cellulose medium. The maximum level of cellulase transcripts, as indicated by the ability to direct biosynthesis of these cellulases in the rabbit reticulocyte cell-free translation system, occurred when the rate of secretion was maximum. This implied that initial regulation, at least, of cellulase biosynthesis occurs at the transcriptional level. There were two distinct mRNA-directed products for each of the cellulases, with sizes estimated to be, for the beta-glucosidase, 95700 and 93800, for the exoglucanase, 59300 and 58200, and for the carboxymethylcellulase, 40600 and 39400. The secreted cellulases are largely glycosylated, as indicated by their binding to concanavalin A and their incorporation of D-[3H]mannose. The labelled protein was fractionated on concanavalin-A-agarose; about 70% of the label was bound. A small amount of each of the cellulases appeared in the unbound fraction; the remainder appeared in fractions eluted with 10 mM methyl glucoside or with 100 mM methyl glucoside plus 500 mM methyl mannoside. These results indicated each of the cellulases had an additional heterogeneity in glycosylation, with the most heavily glycosylated and highest molecular weight form eluting last from the concanavalin-A-agarose. Although tunicamycin (5 micrograms/ml) blocked glycosylation, there was still some secretion but at a reduced rate which was more pronounced for the beta-glucosidase than for the carboxymethylcellulase activity. The size of the tunicamycin-secreted product in each case was, within experimental error, equivalent to that of the mRNA-directed one.