Secretion of a wheat α-amylase expressed in yeast

Abstract

The translocation of secretory proteins across the endoplasmic reticulum involves the recognition and cleavage of an amino-terminal extension called the signal sequence¹. The structure of signal peptides appears to be ubiquitous in having a very hydrophobic central core², so that the signal sequence in secretory proteins from one organism could possibly be recognized by the processing and transport apparatus of another. We therefore wished to investigate whether a protein, α-amylase, one of several hydrolytic enzymes secreted from the aleurone of wheat into the endosperm during germination, could be processed and secreted in an active form from the yeast Saccharomyces cerevisiae, secretion being dependent upon the plant signal sequence. Here, synthesis of α-amylase was by inserting a cDNA clone coding for the entire α-amylase structural gene³ into a yeast expression vector⁴. The α-amylase protein coded for by this gene fusion has the signal sequence located internally, not at the N-terminal end of the polypeptide. Nevertheless, it is processed and the processed form is secreted into the medium in an active form. There are potential industrial applications for yeast that secrete a functional α-amylase.