Conformational transitions of uracil transporter UraA from Escherichia coli: a molecular simulation study

The Escherichia coli uracil/H + symporter UraA, known as the representative nucleobase/cation symporter 2(NCS2) protein, gets involved in several crucial physiological processes for most living organisms on Earth, such as the uptake of nucleobases and transport of vitamin C. Some experiments proposed a working model to explain proton-coupling and uracil transporting process of UraA on the basis of the crystal structure of NCS2 protein, but the details of conformational changes remained unknown. Thus, in order to make clear conformational changes caused by the protonation and deprotonation process of some conserved proton-coupled residues, the molecular dynamics simulation was used to study the conformation of UraA complexes in different protonation states. The results demonstrated that the protonation of residue Glu241 and Glu290 resulted in the whole conformational transition from the inward-open to the outward-open state. It can be concluded that Glu290 was crucial in a network of hydrogen-bonds in the middle of the core domain involving another essential residue, mainly including tyr288 in TM8, Tyr342, Ser338 in TM12, and the network of hydrogen-bonds was the key to maintain the stability of conformation. Protonation of Glu290 affects the stability of network of H-bond and changed the domains TM3 TM10 TM12. Thus, Glu290 may play a vital role as a 'proton trigger' that affects spatial structural of amino and residues near substrate binding side leading to an outward-open conformation transition.