DEVELOPMENTAL REGULATION OF HEXOSAMINE BIOSYNTHESIS BY PROTEIN PHOSPHATASE-2A AND PHOSPHATASE-2C IN BLASTOCLADIELLA-EMERSONII

Extracts of the aquatic fungus Blastocladiella emersonii were found to contain protein phosphatases type 1, type 2A, and type 2C with proper ties analogous to those found in mammalian tissues. The activities of all three protein phosphatases are developmentally regulated, increasi ng during sporulation, with maximum level in zoospores. Protein phosph atases 2A and 2C, present in zoospore extracts, catalyze the dephospho rylation of L-glutamine:fructose-6-phosphate amidotransferase (EC 2.6. 1.16, amidotransferase), a key regulatory enzyme in hexosamine biosynt hesis. The protein phosphatase inhibitor okadaic acid induces encystme nt and inhibits germ tube formation but does not affect the synthesis of the chitinous cell wall. These results strongly suggest that phosph atase 2C is responsible for the dephosphorylation of amidotransferase in vivo. This dephosphorylation is inhibited by uridine-5'-diphospho-N -acetylglucosamine, the end product of hexosamine synthesis and the su bstrate for chitin synthesis. This result demonstrates a dual role of uridine-5'-diphospho-N-acetylglucosamine by inhibiting the activity of the phosphorylated form of amidotransferase and by preventing its dep hosphorylation by protein phosphatases.