The mammalian proton-coupled peptide cotransporter PepT1: sitting on the transporter–channel fence?

The proton-coupled di- and tripeptide transporter PepT1 (SLC15a1) is the major route by which dietary nitrogen is taken up from the small intestine, as well as being the route of entry for important therapeutic (pro)drugs such as the β-lactam antibiotics, angiotensin-converting enzyme inhibitors and antiviral and anti-cancer agents. PepT1 is a member of the major facilitator superfamily of 12 transmembrane domain transporter proteins. Expression studies in Xenopus laevis on rabbit PepT1 that had undergone site-directed mutagenesis of a conserved arginine residue (arginine(282) in transmembrane domain 7) to a glutamate revealed that this residue played a role in the coupling of proton and peptide transport and prevented the movement of non-coupled ions during the transporter cycle. Mutations of arginine(282) to other non-positive residues did not uncouple proton–peptide cotransport, but did allow additional ion movements when substrate was added. By contrast, mutations to positive residues appeared to function the same as wild-type. These findings are discussed in relation to the functional role that arginine(282) may play in the way PepT1 operates, together with structural information from the homology model of PepT1 based on the Escherichia coli lactose permease crystal structure.