A modified procedure for studying enzyme secretion in yeast sphaeroplasts: subcellular distribution of invertase

A significantly modified procedure for investigating enzyme secretion from yeast sphaeroplasts, and results from its application are described. Sphaeroplasts were derepressed for invertase biosynthesis in the presence of helicase and fractionated to reveal the distribution of high and low molecular weight forms of invertase. Secreted enzyme was found to be of high molecular weight, exclusively. Less than 10% of the total invertase activity was present in washed sphaeroplasts and of this, 43% was soluble, consisting of both high and low molecular weight forms of invertase. Washed membranes retained 32% of the internal invertase activity, and on solubilization with Triton X-100 the enzyme was found to be of an intermediate molecular weight. These results are consistent with the hypothesis that invertase is glycosylated at the plasma membrane.