Prepro-alpha-factor has a cleavable signal sequence

Abstract

MAT alpha Saccharomyces cerevisiae secrete a small peptide mating pheromone termed alpha-factor. Its precursor, prepro-alpha-factor, is translocated into the endoplasmic reticulum and glycosylated at three sites. The glycosylated form is the major product in a yeast in vitro translation/translocation system. However, there is another translocated, nonglycosylated product that contains a previously unidentified modification. Contrary to previous results suggesting that the signal sequence of prepro-alpha-factor is not cleaved, amino-terminal radiosequencing has identified this product as prepro-alpha-factor without its signal sequence, that is, pro-alpha-factor. The translocated, glycosylated proteins are also processed by signal peptidase. Moreover, we have found that both purified eukaryotic and prokaryotic signal peptidase can process prepro-alpha-factor. Experiments using a yeast secretory mutant (sec 18) blocked in transport from the endoplasmic reticulum to the Golgi indicate that the protein is also cleaved in vivo. Finally, characterization of the Asn-linked oligosaccharide present on pro-alpha-factor in the yeast in vitro system by use of specific glucosidase and mannosidase inhibitors indicates that they have had the three terminal glucoses and probably one mannose removed. Therefore they most likely consist of Man8GlcNAc2 structures, identical to those found in the endoplasmic reticulum in vivo.