From membrane to molecule to the third amino acid from the left with a membrane transport protein

The lac permease of E. coli is a paradigm for secondary active transporter proteins that transduce the free energy stored in electrochemical ion gradients into work in the form of a concentration gradient. This hydrophobic, polytopic, cytoplasmic membrane protein catalyses the coupled, stoichiometric translocation of β-galactosides and H+, and it has been solubilized, purified, reconstituted into artificial phospholipid vescicles and shown to be solely responsible for β-galactoside transport as a monomer. The lacY gene which encodes the permease has been mechanism of action of these proteins have been conserved throughout evolution. Therefore, studies on bacterial transport proteins which are considerably easier to manipulate than their eukaryotic counterparts have important relevance to transporters in higher order system, particularly with respect to the development of new approaches to structure-function relationships. Although it is now possible to manipulate membrane proteins to an extent that was unimaginable only a few years ago, it is unlikely that transport mechanisms can be defined on a molecular level without information about tertiary structure. As indicated by many of the experiments outlined here, site-directed approaches will be particularly useful for obtaining both static and dynamic information.