CATION AND SUGAR SELECTIVITY DETERMINANTS IN A NOVEL FAMILY OF TRANSPORT PROTEINS

A new family of homologous membrane proteins that transport galactosid es-pentoses-hexuronides (GPH) is described, By analysing the aligned a mino acid sequences of the GPH family, and by exploiting their differe nt specificities for cations and sugars, we have designed mutations th at yield novel insights into the nature of ligand binding sites in mem brane proteins, Mutants have been isolated/constructed in the melibios e transport proteins of Escherichia coli, Klebsiella pneumoniae and Sa lmonella typhimurium, and the lactose transport protein of Streptococc us thermophilus which facilitate uncoupled transport or have an altere d cation and/or substrate specificity, Most of the mutations map in th e amino-terminal region, in or near amphipathic alpha-helices II and I V, or in interhelix-loop 10-11 of the transport proteins. On the basis of the kinetic properties of these mutants, and the primary and secon dary structure analyses presented here, we speculate on the cation bin ding pocket of this family of transporters. The regulation of the tran sporters through interaction with, or phosphorylation by, components o f the phosphoenolpyruvate:sugar phosphotransferase system is also disc ussed.