Structural determination of wild-type lactose permease.
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.
Guan
Lan
L
Department of Physiology, University of California, Los Angeles, CA 90095-1662, USA.
Mirza
Osman
O
Verner
Gillian
G
Iwata
So
S
Kaback
H Ronald
HR
D001665:Binding Sites; D018360:Crystallography, X-Ray / Q000379:methods; D004563:Electrochemistry / Q000379:methods; D004926:Escherichia coli / Q000201:enzymology*; D029968:Escherichia coli Proteins / Q000737:chemistry*; D026901:Membrane Transport Proteins / Q000737:chemistry* / Q000235:genetics*; D008968:Molecular Conformation; D009004:Monosaccharide Transport Proteins / Q000737:chemistry*; D011336:Probability; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D027981:Symporters / Q000737:chemistry*; D014961:X-Ray Diffraction
D029968:Escherichia coli Proteins; C050221:LacY protein, E coli; D026901:Membrane Transport Proteins; D009004:Monosaccharide Transport Proteins; D027981:Symporters; C021499:lactose permease