Detection of inactive precursors of beta-glucanases in Saccharomyces cerevisiae

Accumulation and secretion of exoglucanase activity in yeast secretory mutants

Representative conditional yeast secretory mutants, blocked in transport of secretory and plasma membrane proteins from the endoplasmic reticulum (sec 18), from the Golgi body (sec 7) and in transport of secretory vesicles (sec 1), accumulated exoglucanase, a constitutive yeast activity, when incubated at the restrictive temperature (37 degrees C). Different proportions of the accumulated activity were released by mutant cells under permissive conditions. The presence or absence of cycloheximide during the secretion period made no differences in the results. More than 90% of the internal activity was bound to membrane in wild type cells. However, only the soluble pool underwent changes during the accumulation or secretion periods. The bulk of secretory invertase accumulated by sec 1 was also soluble. By contrast sec 7 and sec 18 accumulated membrane-bound as well as soluble invertase forms and both were secreted in similar proportions in each mutant. More than 90% of the accumulated invertase was secreted at the permissive temperature in sec 18 cells. That percentage was significantly lower for exoglucanase (less than 65%). Concomitantly, invertase accumulated by this mutant exited from the cells with a lower half time (t1/2 = 24 min) than accumulated exoglucanase (t1/2 = 150 min). These results may be interpreted assuming that exoglucanase is exported by a passive flow of the soluble pool.

Accumulation of exoglucanase activity in yeast secretory mutants blocked at the endoplasmic reticulum level

Saccharomyces cerevisiae HMSF-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 degrees 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.

Regulation of beta-exoglucanase activity production by Saccharomyces cerevisiae in batch and continuous culture

The rate of synthesis and secretion of exo-1-3-beta-glucanase activity closely paralleled the specific rate of growth in exponentially growing Saccharomyces cerevisiae cells in batch culture. When the stationary phase was reached both synthesis and secretion stopped. No activity was synthesized when the cells were maintained in carbon sources that did not allow them to grow. Studies in continuous culture indicate a strong relationship between the synthesis of exoglucanase activity and the specific growth rate. These results are taken as evidence of an essential role of this activity during the yeast budding cycle.