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Chitin and Yeast Budding

ALLOSTERIC INHIBITION OF CHITIN SYNTHETASE BY A HEAT-STABLE PROTEIN FROM YEAST

From the ¹ From the National Institute of Arthritis and Metabolic Diseases, National Institutes of Health, Bethesda, Maryland 20014

The soluble fraction of a spheroplast lysate from Saccharomyces carlsbergensis or Saccharomyces cerevisiae, contains a heat-stable, nondialyzable inhibitor of the particulate chitin synthetase. The chitin synthetase inhibitor (CSI) can also be extracted by heating intact cells in buffer. CSI was highly purified by conventional methods of protein fractionation, as indicated by disc gel electrophoresis. The inhibitory power is destroyed by incubation with trypsin, thus confirming that CSI is a protein. CSI is a potent effector, since half-maximal inhibition is attained with less than 1 µg per ml, yet the activity of chitin synthetase is never totally abolished. Kinetically, the inhibition is of the noncompetitive type.

Storage of chitin synthetase results in a decreased sensitivity to CSI, and treatment of the enzyme with trypsin eliminates the effect of CSI altogether, while the enzymatic activity is doubled. The desensitized enzyme is still sedimentable and its K_m for UDP-acetylglucosamine is essentially unchanged.

The content of CSI in yeast increased 3- to 4-fold during growth from the middle logarithmic to the stationary phase. The activity of chitin synthetase also increased. It is postulated that CSI has a regulatory role connected with the budding process since chitin is found in the cell wall at the site where the bud had formed and detached.

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