WHATS NEW WITH LACTOSE PERMEASE

The lactose permease of Escherichia coli is a paradigm for polytopic m embrane transport proteins that transduce free energy stored in an ele ctrochemical ion gradient into work in the form of a concentration gra dient. Although the permease consists of 12 hydrophobic transmembrane domains in probable alpha-helical conformation that traverse the membr ane in zigzag fashion connected by hydrophilic ''loops'', little infor mation is available regarding the folded tertiary structure of the mol ecule. In a recent approach site-directed fluorescence labeling is bei ng used to study proximity relationships in lactose permease. The expe riments are based upon site-directed pyrene labeling of combinations o f paired Cys replacements in a mutant devoid of Cys residues. Since py rene exhibits excimer fluorescence if two molecules are within about 3 .5 Angstrom, the proximity between paired labeled residues can be dete rmined. The results demonstrate that putative helices VIII and IX are close to helix X. Taken together with other findings indicating that h elix VII is close to helices X and XI, the data lead to a model that d escribes the packing of helices VII to XI.