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Saccharomyces cerevisiaeHMSF‐176 (sec 18), a thermosensitive secretory mutant blocked at the endoplasmic reticulum (ER) level, drastically increased its osmotic sensitivity when grown at the restrictive temperature of 37°C in high glucose concentration. This fact led to the erroneous interpretation that glucanases were inactive when localized in the ER. The development of a suitable osmotic stabilizer now indicates that sec 18 accumulates exoglucanase activity. Another ER‐blocked mutant behaved in a similar way. All the accumulated exoglucanase was found in a soluble form. By contrast, a significant portion of the accumulated invertase remained in a membrane‐bound form.

Analysis of exoglucanase activity accumulated by sec mutants from Saccharomyces cerevisiaerevealed the presence of two ionic forms of the major exoglucanase (exo II) secreted into the culture medium. From the accumulation pattern of representative sec mutants and the carbohydrate composition it appears that the less acidic form is converted into the more acidic one by addition of one phosphate to one of the oligosaccharide cores as the enzyme progresses through the secretory pathway. Exoglucanase I, the heavier isoenzyme, was not accumulated by the mutants. Accordingly, it should arise from exoglucanase II after the execution point of sec1 mutation.