2.A.24.1.1
Citrate:Na⁺ symporter, CitS.  Kebbel et al. 2013 presented the three-dimensional map of dimeric CitS obtained with electron crystallography. Each monomer has 13 alpha-helical transmembrane segments; six are organized in a distal helix cluster and seven in the central dimer interface domain. Based on structural analyses and comparison to VcINDY, a molecular model with assigned helices, a model with internal structural symmetry was proposed. Projections of CitS in several conformational states induced by the presence and absence of sodium and citrate as substrates were also proposed. Citrate binding induces a defined movement of alpha helices within the distal helical cluster.  Kebbel et al. 2013 proposed a substrate translocation site and conformational changes that are in agreement with the "alternating access" model. The loop between TMSs VIII and IX folds into an amphipathic surface helix (Sobczak and Lolkema 2005).