title: Kinetic evidence is consistent with the rocker-switch mechanism of membrane transport by GlpT.
abstract: Secondary active transport of substrate across the cell membrane is crucial to many cellular and physiological processes. The crystal structure of one member of the secondary active transporter family, the sn-glycerol-3-phosphate (G3P) transporter (GlpT) of the inner membrane of Escherichia coli, suggests a mechanism for substrate translocation across the membrane that involves a rocker-switch-type movement of the protein. This rocker-switch mechanism makes two specific predictions with respect to kinetic behavior: the transport rate increases with the temperature, whereas the binding affinity of the transporter to a substrate is temperature-independent. In this work, we directly tested these two predictions by transport kinetics and substrate-binding experiments, integrating the data on this single system into a coherent set of observations. The transport kinetics of the physiologically relevant G3P-phosphate antiport reaction were characterized at different temperatures using both E. coli whole cells and GlpT reconstituted into proteoliposomes. Substrate-binding affinity of the transporter was measured using tryptophan fluorescence quenching in detergent solution. Indeed, the substrate transport velocity of GlpT increased dramatically with temperature. In contrast, neither the apparent Michaelis constant (Km) nor the apparent substrate-binding dissociation constant (Kd) showed temperature dependence. Moreover, GlpT-catalyzed G3P translocation exhibited a completely linear Arrhenius function with an activation energy of 35.2 kJ mol-1 for the transporter reconstituted into proteoliposomes, suggesting that the substrate-loaded transporter is delicately poised between the inward- and outward-facing conformations. When these results are taken together, they are in agreement with a rocker-switch mechanism for GlpT.
authors.affiliation: Kimmel Center for Biology and Medicine at the Skirball Institute of Biomolecular Medicine and Department of Cell Biology, New York University School of Medicine, 540 First Avenue, New York, New York 10016, USA.
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Law
authors.forename: Qiang
authors.initials: Q
authors.lastname: Yang
authors.forename: Celine
authors.initials: C
authors.lastname: Soudant
authors.forename: Peter C
authors.initials: PC
authors.lastname: Maloney
authors.forename: Da-Neng
authors.initials: DN
authors.lastname: Wang
mesh_terms: D007700:Kinetics; D026901:Membrane Transport Proteins / Q000737:chemistry / Q000378:metabolism*; D011485:Protein Binding; D011487:Protein Conformation; D021381:Protein Transport; D011510:Proteolipids
chemical_list: C488962:GlpT protein, E coli; D026901:Membrane Transport Proteins; D011510:Proteolipids; C020485:proteoliposomes