The biosynthesis of beta(1 to 3)glucan, the major structural component of the yeast cell wall, is studied to understand morphogenesis of the wall. It has now been found that the GTP-binding protein that regulates the activity of beta(1 to 3)glucan synthase is Rhol protein, a member of the Ras superfamily. rho1 mutants are defective in glucan synthase and the defect can be corrected by recombinant Rho1p or by our purified preparations of GTP-binding protein. Since the activity of Rho1p depends on the amount of GTP bound to it , this finding suggests that glucan synthase (hence wall synthesis) can be switched on and off at different points of the cell cycle by modulating the protein-bound nucleotide. To understand aspects of the yeast cell wall architecture, the structure of a complex obtained by partial degradation of the wall with chitinase and beta(1 to 3)glucanase is studied. This complex contains mannoprotein, a(16)glucan, and some glycosyl residues of chitin and beta(1 to 3)glucan that were spared by the action of the hydrolytic enzymes. Analysis of the material indicates that it represent a linkage region between chitin and beta(1 to 3)glucan. The core of this region consist of mannoprotein, to which chitin and beta(1 to 3)glucan are linked through beta(1 to 6)glucan bridges. This novel structure differs from one previously studied in our laboratory, in which chitin and glucan are directly attached to each other.
