Abstract
The TRG1/PDI1 gene of Saccharomyces cerevisiae is essential for growth and encodes a lumenal endoplasmic reticulum (ER) glycoprotein that is structurally related to thioredoxin and is involved in the secretory pathway. We have tested whether the yeast Trg1/Pdi1 protein can be replaced in vivo by three members of the mammalian thioredoxin-related protein family, protein disulfide isomerase (PDI), ERp72, and ERp61. Multicopy plasmids containing galactose-inducible rodent PDI and ERp72 genes support germination and growth of haploid trg1 null mutants in galactose-containing media, whereas the ERp61 gene is inactive. Strains expressing PDI or ERp72 instead of Trg1 are thermosensitive. An overproduced mutant Trg1 protein lacking the HDEL retention signal supports growth, whereas a truncated version of the protein containing only one thioredoxin-like domain is inactive. The mammalian proteins were localized to both the soluble and microsomal membrane fraction of yeast cells. Our observations indicate that the two unglycosylated mammalian proteins PDI and ERp72 are capable of replacing at least some of the critical functions of Trg1, in spite of the fact that the three proteins diverge considerably in sequences surrounding the thioredoxin-related domains.
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