Inactivation and chemical alteration of mating factor α by cells and spheroplasts of yeast

Abstract

Mating factor .alpha. isolated from yeast [Saccharomyces cerevisiae] culture filtrates was radiolabeled by lactoperoxidase-catalyzed iodination, with full retention of biological activity. The 125I-labeled .alpha. factor bound at low levels to cells of both mating types (a and .alpha.) but not to spheroplasts. Despite the low level of binding, large quantities of .alpha. factor activity were lost by incubation with a cells and a spheroplasts, but not with .alpha. or a/.alpha. diploid cells. The amount of activity removed from the culture medium was much larger than the amount of 125I-labeled .alpha. factor bound to the cells and was correlated with the appearance of radiolabeled derivatives separable by TLC. Upon removal of the cell wall of .alpha. and a/.alpha. cells, the spheroplasts acquired the ability to remove .alpha. factor activity from culture medium, to generate derivatives of .alpha. factor, and to respond to .alpha. factor by a morphological alteration resembling the response of a cells. These findings raise the possibility that the specific enzyme capable of altering .alpha. factor, possibly a peptidase, is associated with both a and .alpha. cells but is masked by the .alpha. cell wall. This suggestion is consistent with the observation that the .alpha. factor activities of G1 arrest and cell elongation were blocked by preincubation of a cells with the protease inhibitor Trasylol.