Acid protease from Trichoderma reesei: limited proteolysis of fungal carbohydrases

Mechanisms regulating post-secretory limited proteolysis, carried out by th e acid protease from Trichoderma reesei, were studied by following the rele ase of alpha-galactosidase and multiple forms of cellobiohydrolase from thi s species. Both the rate of the proteolysis and the mode of action of the p rotease were affected by the pH of the culture medium, and only weakly depe nded on the amount of the enzyme. At pH between 2.7 and 3.5 the proteolytic reaction was limited, while at lower pH proteins were completely digested. Proteolysis depended on the degree of glycosylation of secreted enzymes. I nhibition of gost-secretory deglycosylation decreased the rate of limited p roteolysis in the culture medium in the course of fungal growth. Glucose an d cellobiose, the main products of cellulose degradation carried out by the fungal cellulolytic complex, inhibited the proteolysis of the cellobiohydr olase in a concentration-dependent manner. A 32-kDa aspartic protease (EC 3 .4.23.18) secreted by T. reesei was purified to homogeneity. The acid prote ase cleaved alpha-galactosidase and cellobiohydrolase into the same proteol ytic fragments that had been isolated from the culture medium.