Expected location of C355 and comparison of the 12–13 loops from vSGLT and hSGLT1.

A sequence alignment between vSGLT and hSGLT [12] showed that the position of C355 in hSGLT1 corresponds to N328 in vSGLT. Using vSGLT crystal structure (PDB# 3DH4) as a template, C355 would be located between TMS7 and 8, more precisely between extracellular helix 7a and 7b that are colored in yellow in the present figure (LeuT numbering of TMS’s). A top view shows that C355 would be located 10Ǻ away from the closest membrane-cotransporter interface. The figure also shows the difference in the loop linking TMS12 (in pink) and TMS13 (in purple). The 12–13 loop of vSGLT (in magenta) is 24 amino acids long and the corresponding loop in hSGLT1 (in blue) is 90 amino acids long. The position of E624 in the hSGLT1 12–13 loop is indicated. When mutated to a cysteine, that position can be labelled from the extracellular solution and hVoS FRET quenching suggested that this position should be located clearly above the membrane center.