title: Identification of the dimer interface of the lactose transport protein from Streptococcus thermophilus.
abstract: The lactose transporter from Streptococcus thermophilus catalyses the symport of galactosides and protons. The carrier domain of the protein harbours the contact sites for dimerization, and the individual subunits in the dimer interact functionally during the transport reaction. As a first step towards the elucidation of the mechanism behind the cooperation between the subunits, regions involved in the dimer interface were determined by oxidative and chemical cross-linking of 12 cysteine substitution mutants. Four positions in the protein were found to be susceptible to intermolecular cross-linking. To ensure that the observed cross-links were not the result of randomly colliding particles, the cross-linking was studied in samples in which either the concentration of LacS in the membrane was varied or the oligomeric state was manipulated. These experiments showed that the cross-links were formed specifically within the dimer. The four regions of the protein located at the dimer interface are close to the extracellular ends of transmembrane segments V and VIII and the intracellular ends of transmembrane segments VI and VII.
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D001692:Biological Transport; D003432:Cross-Linking Reagents / Q000494:pharmacology; D003545:Cysteine / Q000737:chemistry; D003593:Cytoplasm / Q000378:metabolism; D003902:Detergents / Q000494:pharmacology; D019281:Dimerization; D004305:Dose-Response Relationship, Drug; D004591:Electrophoresis, Polyacrylamide Gel; D004926:Escherichia coli / Q000378:metabolism; D029968:Escherichia coli Proteins; D005033:Ethylmaleimide / Q000494:pharmacology; D015151:Immunoblotting; D026901:Membrane Transport Proteins / Q000737:chemistry*; D008969:Molecular Sequence Data; D009004:Monosaccharide Transport Proteins; D009154:Mutation; D011485:Protein Binding; D020836:Protein Structure, Quaternary; D017434:Protein Structure, Tertiary; D011522:Protons; D013291:Streptococcus / Q000378:metabolism*; D027981:Symporters; D013997:Time Factors
chemical_list: D003432:Cross-Linking Reagents; D003902:Detergents; D029968:Escherichia coli Proteins; C050221:LacY protein, E coli; D026901:Membrane Transport Proteins; D009004:Monosaccharide Transport Proteins; D011522:Protons; D027981:Symporters; C021499:lactose permease; D003545:Cysteine; D005033:Ethylmaleimide