Asymmetric stability among the transmembrane helices of lactose permease.
Combining structure determinations from nuclear magnetic resonance (NMR) data and molecular dynamics simulations (MD) under the same environmental conditions revealed a startling asymmetry in the intrinsic conformational stability of secondary structure in the transmembrane domain of lactose permease (LacY). Eleven fragments, corresponding to transmembrane segments (TMs) of LacY, were synthesized, and their secondary structure in solution was determined by NMR. Eight of the TMs contained significant regions of helical structure. MD simulations, both in DMSO and in a DMPC bilayer, showed sites of local stability of helical structure in these TMs, punctuated by regions of conformational instability, in substantial agreement with the NMR data. Mapping the stable regions onto the crystal structure of LacY reveals a marked asymmetry, contrasting with the pseudosymmetry in the static structure: the secondary structure in the C-terminal half is more stable than in the N-terminal half. The relative stability of secondary structure is likely exploited in the transport mechanism of LacY. Residues supporting proton conduction are in more stable regions of secondary structure, while residues key to substrate binding are found in considerably unstable regions of secondary structure.
Bennett
Michael
M
Department of Molecular and Cell Biology, University of Connecticut, Storrs, Connecticut 06269, USA.
D'Rozario
Robert
R
Sansom
Mark S P
MS
Yeagle
Philip L
PL
D000595:Amino Acid Sequence; D009682:Magnetic Resonance Spectroscopy; D026901:Membrane Transport Proteins / Q000737:chemistry* / Q000378:metabolism; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010446:Peptide Fragments / Q000138:chemical synthesis / Q000737:chemistry / Q000378:metabolism; D017433:Protein Structure, Secondary
D026901:Membrane Transport Proteins; D010446:Peptide Fragments; C021499:lactose permease