Yeast Exoglucanases. Where Redundancy Implies Necessity

Three exoglucanase genes have been described in Saccharomyces cerevisiae. The bulk of the exoglucanase (EXO) activity is encoded by the EXG1 gene, whose primary gene product is differentially glycosylated during its transit to the cell surface to yield three isoenzymes: EXOI, EXOII and EXOII1/2. EXOII, the major isoenzyme, carries two short oligosaccharides, each one consisting of an inner core with a single branch of the outer chain, attached to both potential glycosylation sites present in the molecule (Asn165 and Asn325). EXOI and EXOII1/2 are minor representatives. The second carries a single short sugar residue attached to Asn165 whereas the former elongate the outer chain of, at least, one oligosaccharide as other cell wall mannoproteins. The protein portion of the EXGI gene product is cleaved in Golgi by the Kex2 protease. A different exoglucanase, encoded by a second gene (EXG2), has been characterized as a heavily glycosylated, membrane bound 200 kDa glycoprotein. Finally, a third exoglucanase, encoded by the SSG gene, is synthesized during sporulation of diploids. Exoglucanases similar to those encoded by the EXG1 gene have been detected in other yeasts and characterized in depth in Candida albicans. The three polypeptides from S. cerevisiae and its counterpart from C. albicans have several conserved regions occupying the same relative positions. Studies on the function of these highly conserved enzymes are rather inconclusive.