Isolation of new temperature‐sensitive mutants of Saccharomyces cerevisiae deficient in mannose outer chain elongation

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Abstract
We have isolated two temperature‐sensitive Saccharomyces cerevisiae mutants which exhibit a deficiency in mannose outer chain elongation of asparagine‐linked oligosaccharide. The size of yeast glycoprotein, secretery form of invertase, of one mutant (ochl) was slightly larger than that of the sec18 mutant at the non‐permissive temperature, while that of the other mutant (och2) was almost the same as that of the sec18 mutant. Unlike sec mutants, the och mutants were not deficient in secretion of invertase. The och1 mutant showed a 2+:2− cosegregation with regard to the temperature sensitivity and mannose outer chain deficiency, suggesting that a single gene designated as OCHI is responsible for these two phenotypes. The och1 mutant stopped its growth at the early stage of bud formation and rapidly lost its viability at the non‐permissive temperature. The och1 mutation was mapped near the ole1 on the left arm of chromosome VII. The och1 mutant cells accumulated the external invertase containing a large amount of core‐like oligosaccharides (Man9–10GlcNAc2) and a small amount of high mannose oligosaccharides (>Man50GlcNAc2) at the non‐permissive temperature. Production of the active form of human tissue‐type plasminogen activator was increased in the och1 mutant compared with the parental strain, suggesting the potential advantage of this mutant for the production of mammalian‐type glycoproteins which lack mannose outer chains in yeast.