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

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Abstract
Mating factor α isolated from yeast culture filtrates was radiolabeled by lactoperoxidase-catalyzed iodination, with full retention of biological activity. The 125 I-labeled α factor bound at low levels to cells of both mating types ( a and α) but not to spheroplasts. Despite the low level of binding, large quantities of α factor activity were lost by incubation with a cells and a spheroplasts, but not with α or a /α diploid cells. The amount of activity removed from the culture medium was much larger than the amount of 125 I-labeled α factor bound to the cells and was correlated with the appearance of radiolabeled derivatives separable by thin-layer chromatography. Upon removal of the cell wall of α and a /α cells, the spheroplasts acquired the ability to remove α factor activity from culture medium, to generate derivatives of α factor, and to respond to α factor by a morphological alteration resembling the response of a cells. These findings raise the possibility that the specific enzyme capable of altering α factor, possibly a peptidase, is associated with both a and α cells but is masked by the α cell wall. This suggestion is consistent with the observation that the α factor activities of G 1 arrest and cell elongation were blocked by preincubation of a cells with the protease inhibitor Trasylol.