The beta-glucosidase of the yeast cell surface.

There are two distinct components of the system which limits the rate at which intact cells of S. cerevisiae C hydrolyze external beta-glucosides; one component requires metabolic energy and the other is stereospecific for beta-glucosides. The stereospecific component is localized at the cell membrane, as shown by its sensitivity to heavy metal inhibitors which did not penetrate the cell under the conditions used. It was shown that cellobiose-grown cells were able to remove cellobiose from the medium in which they were incubated, and that the cellobiose uptake system was identical to that which limits the patent beta-glucosidase activity. In order to test the hypothesis that the system in question was a transport system, for beta-glucosides the ability of cellobiose-grown cells to take up (14)C-labeled methyl-beta-glucoside (MBG) was studied. The induced cells were able to take up MBG-(14)C and the label could be partially chased out by cold MBG and cellobiose; glucose-grown cells could not incorporate label. However, induced cells could not take up label when incubated with (14)C-MBG, thus excluding the hypothesis of transport of intact beta-glucosides. It was concluded that the stereospecific membrane component was actually a beta-glucosidase, coupled to an energy-dependent transport system for the glucose moiety; the function of the latter was rate-limiting in the over-all activity of the entire system.