TOPOGRAPHICAL ANALYSIS OF THE PLASMA MEMBRANE-ASSOCIATED SUCROSE BINDING-PROTEIN FROM SOYBEAN

Plasma membranes of soybean cells actively engaged in sucrose transpor t have a sucrose binding protein (SBP) that does not appear to be an i ntegral membrane protein. Experiments were undertaken to analyze the t opographical association of this protein with the membrane. Treatment of purified plasma membrane vesicles with either 1 M KCl or KI release d less than 35% of the sucrose binding protein from the membrane where as treatment with either 4 M urea or 0.1 M Na2CO3, pH 11.5, disassocia ted between 50 and 70%, respectively, of this protein from the membran e. SDS, at either 0.5x, 1x, or 10x of its critical micelle concentrati on, effectively solubilized the sucrose binding protein. The nonionic detergents Triton X-100 and CHAPS, at either 0.5x, 1x, or 10x of their critical micelle concentration, solubilized between 65 and 75% of thi s protein. When either native plasma membrane-associated or in vitro-t ranscribed and -translated SBP were subjected to Triton X-114 phase se paration, 80% partitioned into the detergent-poor aqueous phase. These results indicate that the SBP is a peripheral membrane protein but al so suggest that there is a population of this protein that is tethered to the membrane.