A novel, small endoglucanase gene, egl5, from Trichoderma reesei isolated by expression in yeast

A method is presented for the isolation of genes encoding hydrolytic enzymes without any knowledge of the corresponding proteins. cDNA made from the organism of interest is cloned into a yeast vector to construct an expression library in the yeast Saccharomyces cerevisiae. Colonies producing hydrolytic enzymes are screened by activity plate assays. In this work, we constructed a yeast expression library from the filamentous fungus Trichoderma reesei and isolated a new beta-1,4-endoglucanase gene on plates containing beta-glucan. This gene, egl5, codes for a previously unknown small protein of 242 amino acids. Despite its small size, the protein contains two conservative domains found in Trichoderma cellulases, namely the cellulose-binding domain (CBD) and the linker region that connects the CBD to the catalytic core domain. Molecular modelling of the EGV CBD revealed some interesting structural differences compared to the CBD of the major cellulase CBHI from T. reesei. The catalytic core of EGV is unusually small for a cellulase and represents a new family of cellulases (Family K) and of glycosyl hydrolases (Family 45) together with the endoglucanase B of Pseudomonas fluorescens and the endoglucanase V of Humicola insolens on the basis of hydrophobic cluster analysis.