Structural basis for substrate binding and specificity of a sodium/alanine symporter AgcS
The amino acid, polyamine, and organocation (APC) superfamily is the second largest superfamily of membrane proteins forming secondary transporters that move a range of organic molecules across the cell membrane. Each transporter in APC superfamily is specific for a unique sub-set of substrates, even if they possess a similar structural fold. The mechanism of substrate selectivity remains, by and large, elusive. Here we report two crystal structures of an APC member from Methanococcus maripaludis, the alanine or glycine:cation symporter (AgcS), with L- or D-alanine bound. Structural analysis combined with site-directed mutagenesis and functional studies inform on substrate binding, specificity, and modulation of the AgcS family and reveal key structural features that allow this transporter to accommodate glycine and alanine while excluding all other amino acids. Mutation of key residues in the substrate binding site expand the selectivity to include valine and leucine. Moreover, as a transporter that binds both enantiomers of alanine, the present structures provide an unprecedented opportunity to gain insights into the mechanism of stereo-selectivity in APC transporters.