Trypsin-uncoupled synthesis and secretion of yeast invertase: implications for the mechanism of secretion.

The subcellular distribution of invertase was examined after synthesis and secretion by sphaeroplasts had been uncoupled by the addition of 30 microgram mL-1 trypsin. Sphaeroplasts secreted only the high molecular weight invertase during uncoupling by trypsin. The level of low molecular weight, 'small' invertase in the soluble internal pool was found to be elevated by over fivefold, and the membrane-associated pool was found to contain low molecular weight invertase in addition to intermediate molecular weight invertase, after 1.5 h of trypsin treatment. Purified plasma membranes from trypsin-treated sphaeroplasts had no detectable mannan synthetase activity. On the basis of these and previous findings, a working hypothesis wherein invertase is synthesized on the internal surface of the plasma membrane and glycosylated during its transit to the external surface is presented.