On the kinetics and conformational dynamics of elevator transporters

In this thesis are collected information about the transport kinetics and dynamics of GltTk and CitS, two secondary active membrane transporters which translocate aspartate and citrate respectively along the sodium gradient. In proteins that translocate substrates following steady state kinetics, such as GltTk, the analysis of substrate transport rates in a wide range of substrate and co-transported ions allow the calculation of elusive kinetic parameters. In proteins that follow rapid equilibrium kinetics, such as CitS, the rates can be analyzed to extrapolate the order of binding of substrates and co-ions. In addition, an HS-AFM study carried out on CitS revealed the presence of three meta-stable protein conformations that occur during transport. In chapter 5, the crystal structure of GltTk in complex with the non-canonical amino acid D-aspartate reveals the binding mode of the substrate's stereoisomer. Finally the role of a conserved methionine residue which is putatively linked to coordinating a sodium ion in GltTk has been investigated by measuring the transport stoichiometry on several mutants.