Direct evidence that maltose transport activity is affected by the lipid composition of Brewer's yeast

A brewer's yeast strain was grown with maltose as sole carbon source under strictly anaerobic conditions with and without ergosterol and/or unsaturated fatty acid (Tween 80) supplements. Under all these conditions the MALO genes for maltose transporters were strongly expressed during growth. The fatty acid unsaturation indices of growing and stationary phase yeast were increased from about 20% to 56-69% by supplementation with Tween 80. Ergosterol contents were increased up to at least 4-fold by supplementation with ergosterol and Tween 80. Maltose transport activity measured at 20 degrees C was not increased by supplementation with Tween 80 alone, but was increased 2-fold and 3-fold, respectively, in growing and stationary phase yeast by supplementation with ergosterol together with Tween 80. The stimulation of maltose transport by ergosterol was greater when the transport was measured at temperatures (10 degrees C and 0 degrees C) lower than 20 degrees C. The results show that proper function of maltose transporters requires adequate amounts of ergosterol in the yeast. This effect may partly explain the low maltose (and maltotriose) uptake rates both in the second half of brewery fermentations, when the sterol content of yeast has fallen, and when fresh wort is pitched with sterol-deficient cropped yeast.