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Membrane transport proteins transduce free energy stored in electrochemical ion gradients
into a concentration gradient and are a major class of membrane proteins, many of which
play important roles in human health and disease. Recently, the X-ray structure of the
Escherichia coli lactose permease (LacY), an intensively studied member of a large group
of related membrane transport proteins, was solved at 3.5 Å.
LacY is composed of N- and C-terminal domains, each with six transmembrane helices,
symmetrically positioned within the molecule. The structure represents the inward-facing
conformation, as evidenced by a large internal hydrophilic cavity open to the cytoplasmic
side. The structure with a bound lactose homolog reveals the sugar-binding site in the
cavity, and a mechanism for translocation across the membrane is proposed in which the
sugar-binding site has alternating accessibility to either side of the membrane.
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