The kamikaze approach to membrane transport

Abstract

Membrane transport proteins mediate the movement of molecules into, or out of, cells, intracellular organelles and across epithelia. Despite their abundance in the genome and evident importance for the cell, we know little about their structure and function, largely because their hydrophobic and metastable nature makes them difficult to study. Recent developments have allowed initial insight into the structure and mechanism of membrane transport proteins. One example is the lactose permease from Escherichia coli, a member of the major facilitator superfamily. This membrane protein uses free energy released from the energetically downhill translocation of H⁺ in response to an electrochemical H⁺ gradient to drive the accumulation of specific sugars against a concentration gradient. Extensive use of site-directed mutagenesis demonstrates that only six amino acid residues are irreplaceable with respect to active lactose transport. Furthermore, mutants engineered for various biochemical and biophysical approaches provide structural information about how the helices are packed and how the irreplaceable residues interact to catalyse transport. Through this work, charge pairs have been identified that mediate substrate binding and H⁺ translocation. The residues that are irreplaceable for activity are conserved in other members of the oligosaccharide/H⁺ symport subfamily, but are not found in other members of the major facilitator superfamily. Despite this, it is thought that relatively few residues will be critical for transport in these other families of membrane transport proteins and that the conformational changes involved will be largely rigid body movements of the transmembrane helices.