ISOLATION OF A TRICHODERMA-REESEI CDNA-ENCODING GTP - ALPHA-D-MANNOSE-1-PHOSPHATE GUANYLTRANSFERASE INVOLVED IN EARLY STEPS OF PROTEIN GLYCOSYLATION

A cDNA coding for GTP:alpha-D-mannose-1-phosphate guanyltransferase (M PG1 transferase) (EC 2.7.7.13) was isolated from a cDNA library of the Trichoderma reesei RutC-30 strain by suppression of the yeast Sacchar omyces cerevisiae mutation in the DPM1 gene encoding mannosylphosphodo lichol (MPD) synthase. The nucleotide sequence of the 1.6 kb-long cDNA revealed an ORF which encodes a protein of 364 amino acids. Sequence comparisons demonstrate 70% identity with the S. cerevisiae guanyl tra nsferase gene (MPG1) and 75% identity with the Schizosaccharomyces pom be homologue. No similarity was found with the MPD synthase encoded by the S. cerevisiae DPM1 gene. The possibility that cloned cDNA encodes a product with a MPD synthase activity was also excluded by transform ing a heterozygous S. cerevisiae dpm1::LEU2/DPM1 diploid, which did no t lead to the restoration of viability of the dpm1 spores. Simultaneou sly,, a significant increase in MPG transferase activity, as compared with the wild-type yeast, was observed in cellular extracts when the m pg1 cDNA from Trichoderma was expressed in the S. cerevisiae dpm1-6 mu tant.