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Structural determination of wild-type lactose permease

2007; National Academy of Sciences; Volume: 104; Issue: 39 Linguagem: Inglês

10.1073/pnas.0707688104

1091-6490

Lan Guan, Osman Mirza, G. Verner, So Iwata, H. Ronald Kaback,

Bacterial Genetics and Biotechnology

Here we describe an x-ray structure of wild-type lactose permease (LacY) from Escherichia coli determined by manipulating phospholipid content during crystallization. The structure exhibits the same global fold as the previous x-ray structures of a mutant that binds sugar but cannot catalyze translocation across the membrane. LacY is organized into two six-helix bundles with twofold pseudosymmetry separated by a large interior hydrophilic cavity open only to the cytoplasmic side and containing the side chains important for sugar and H(+) binding. To initiate transport, binding of sugar and/or an H(+) electrochemical gradient increases the probability of opening on the periplasmic side. Because the inward-facing conformation represents the lowest free-energy state, the rate-limiting step for transport may be the conformational change leading to the outward-facing conformation.