Molecular biology of yeast exoglucanases

Three exoglucanase (Exg) genes have been reported in Saccharomyces cerevisiae. Gene EXG1 encodes the major isoenzyme (ExgI). Differential glycosylation of the primary translation product throughout the secretory pathway results in the secretion of several glycoforms. The major glycoform (ExgIb) contains two short carboxypeptidase Y-like oligosaccharides attached to both potential glycosylation sites present in the molecule. A minor glycoform (ExgIa) arises from the former by elongation of the second oligosaccharide. The protein portion is processed in the secretory pathway by the Kex2 protease. Gene EXG2 encodes a 63 kDa polypeptide with 12 potential glycosylation sites. The predicted protein, ExgII, carries a signal peptide at the amino terminus and a glycosyl-phosphatidyl inositol anchoring motif at the carboxyl end. The latter appears responsible for the particulate nature of this isoenzyme, since its elimination results in the secretion of this activity into the culture medium. Gene SSG1 encodes a 52 kDa polypeptide which is specifically synthesized during sporulation of diploids. SSG1 expression is under control of both sexual (a1-alpha 2 element) and nutritional control. Although homozygous ssg1/ssg1 diploid strains are still able to complete sporulation, they exhibited a delay in the appearance of mature asci. Single or double disruption of EXG1 and EXG2 did not result in any relevant phenotype and the triple mutant behaved as ssg1/ssg1. A ExgI-related enzyme is secreted by Candida albicans. All these four enzymes share 8 highly conserved regions in the same relative positions, indicating that they derived from a common ancestor. However, no clear function has so far been demonstrated for them.