Regulation of chitin synthase activity in the dimorphic fungus Benjaminiella poitrasii by external osmotic pressure.

The effects of changes in external osmotic pressure on chitin synthase activity of a dimorphic fungus, Benjaminiella poitrasii, have been investigated. Mycelial and yeast cells incubated in medium of low osmolality (distilled water, 0 mOsm) for 10 min had 2-3-fold higher specific activities of native chitin synthase in mixed membrane preparations than cells that had been subjected to a high osmolality medium (1.2 M sorbitol in distilled water, 1612 mOsm). Cells suspended in media of different osmolalities for 10 min were also affected in the extent of germ tube formation. Germ tube formation was highest in cells incubated in low osmolality medium. The addition of protein phosphatase inhibitors (cyclosporin A, 1.2 micrograms/ml; cantharidin, 20 microM) abolished the effect of hypo-osmotic stress on chitin synthase activation of yeast mixed membrane preparations. The presence of protein kinase inhibitors (genistein, 40 micrograms/ml; H-7, 100 microM) and a Ca2+ channel blocker (verapamil, 50 microM) reduced chitin synthase activity to 50-60% of that observed in cells under hypo-osmotic shock. These inhibitors also inhibited germ tube formation. This suggests that chitin synthase activity and yeast hyphal morphogenesis are both subject to regulation by osmotic pressure, phosphorylation and calcium.