[2] The solutes transported may be sugars, amino acids, organo cations such as choline, nucleosides, inositols, vitamins, urea or anions, depending on the system.

Sodium/substrate symport (or co-transport) is a widespread mechanism of solute transport across cytoplasmic membranes of pro- and eukaryotic cells.

A conserved tyrosine in the first transmembrane segment of solute:sodium symporters is involved in Na+-coupled substrate co-transport.

A 13 TMS topology with a periplasmic N-terminus and a cytoplasmic C-terminus has been experimentally determined for the proline:Na+ symporter, PutP, of E.

However, MctP of Rhizobium leguminosarum may take up monocarboxylates via an H+ symport mechanism as a dependency on Na+ could not be demonstrated and uptake was strongly inhibited by 10 μM CCP.

Homologous regulatory domains are found in Agrobacterium, Mesorhizobium, Sinorhizobium, Vibrio cholerae and Bacillus species.

The formation of the ternary complex induces another structural change that exposes sodium and substrate to the other site of the membrane.

Substrate and sodium are released, and the empty transporter re-orientates in the membrane, allowing the cycle to start again.