COMPUTER-ASSISTED NONLINEAR-REGRESSION ANALYSIS OF THE MULTICOMPONENTGLUCOSE-UPTAKE KINETICS OF SACCHAROMYCES-CEREVISIAE

The kinetics of glucose uptake in Saccharomyces cerevisiae are complex . An Eadie-Hofstee (rate of uptake versus rate of uptake over substrat e concentration) plot of glucose uptake shows a nonlinear form typical of a multicomponent system. The nature of the constituent components is a subject of debate. It has recently been suggested that this nonli nearity is due to either a single saturable component together with fr ee diffusion of glucose or a single constitutive component with a vari able K-m, rather than the action of multiple hexose transporters. Gene tic data support the existence of a family of differentially regulated glucose transporters, encoded by the I;CYT genes. In this work, kinet ic expressions and nonlinear regression analysis, based on an improved zero trans-influx assay, were used to address the nature of the compo nents of the transport system. The results indicate that neither one c omponent with free diffusion nor a single permease with a variable K-m can explain the observed uptake rates. Results of uptake experiments, including the use of putative alternative substrates as inhibitory co mpounds, support the model derived from genetic analyses of a multicom ponent system with at least two components, one a high-affinity carrie r and the other a low-affinity carrier. This approach was extended to characterize the activity of the SNF3 protein and identify its role in the depression of high-affinity uptake. The kinetic data support a ro le of SNF3 as a regulatory protein that may not itself be a transporte r.