All living cells are enclosed by a selectively permeable membrane that serves as a barrier between the external environment and the contents of the cell. This membrane consists of a lipid bilayer that is not permeable to most of molecules, especially to polar substances. Ions and water-soluble molecules require the assistance of specific protein carriers to transport them across the membrane. These integral membrane proteins provide passive or active transport of their substrates when the molecules move down or against their concentration gradient respectively. Active transport can be energized from ATP hydrolysis (by ATP-binding cassette (ABC) transporters) or by utilizing the electrochemical gradient of a second substrate (co- or anti-transported by so-called secondary active transporters).
In this work we study mechanisms of active uptake of amino acids by the bacterial transporter GlnPQ and the archaeal transporter GltT using X-ray crystallography. GlnPQ is an ABC-importer with soluble substrate-binding domains that have different affinities for asparagine, glutamine and glutamate [1-3]. Archaeal GltT is a secondary transporter that catalyses the uptake of aspartate together with sodium ions [4-6]. Several structures of these transporters are already known, however some details of their functioning remain elusive. Crystallization and structural characterization of these transporters in different states with high resolution is needed to clarify the existing structural models and results of biochemical experiments.
This work is supported by the grants from the Netherlands Organisation for Scientific research (NWO, Top-subsidy grant 700.56.302 to BP and VICI grant 865.11.001 to DS) and by European Research Council ERC starting grant 282083 to DS.