Membrane transport in an osmotically fragile mutant of Saccharomyces cerevisiae

Transport properties of the osomotically fragile strain VY1160 of saccharomyces cerevisiae were compared with those of the parent S288c strain. Mediated diffusion of 6‐deoxy‐D‐glucose was practically unaffected; membrane‐potential dependent transport of D‐glucosamine was very much depressed in the fragile strain. The H+ ‐driven transport of L‐lysine and Lproline, as well as that of the hitherto uninvestigated D‐glucose‐6‐phosphate, were also very depressed. 2‐Deoxy‐D‐glucose transport displayed slightly different kinetic parameters. Primary H+ extrusion by the plasma membrane H‐ATPase was not diminished althpough the ATP‐splitting activity was depressed by about 50%. The overall proton‐motive force (pmf) of the fragile mutant at pH 5.5 was only m V while in the parent strain it was 108 m V. In parallel with this, spontaneous acidfication of the external medium to stimulate (a CO2‐associated event) was only about 2% of that in the parent strain. The defect in his, together with the inability to stimulate transport protein synthesis by glucose, may account for the generally poorer transport performance of the fragile mutant.