Evidence for facilitated transport in the absorption of sterols by Saccharomyces cerevisiae.

Saccharomyces cerevisiae is known to absorb sterols readily in the absence of air. As shown in this paper, yeast cells also will absorb sterols with and without various double bonds or an alkyl group at C-24 in the presence of air at a concentration (ca. 10% of the gas phase) which is growth-limiting due to limited sterol synthesis. However, if the growth conditions are changed to be fully aerobic, sterol is no longer absorbed to any significant extent even when the sterol in the medium (ergosterol) is the same as that present in the cells. This implies that sterol in the medium does not equilibrate passively with sterol in the plasma membrane and that some sort of facilitated transport, which can be turned on and off, is responsible for the entry of sterol when it occurs as a response to an inadequate endogenous supply of sterol. In agreement with facilitated transport mediated by protein binding, yeast cells in an auxotrophic state for sterol exhibit a high degree of stereoselectivity with respect to the orientation of the side chain around the C-17(20)-bond. For instance, E-17(20)- but not Z-17(20)-dehydrocholesterol is absorbed by cells undergoing limited growth with 10% air.