[Molecular characteristics and tissue distribution of peptide transporters]

At present, there are at least five kinds of peptide transporters identified in animal tissues, in which PepT1 and PepT2 were discussed most extensively. PepT1 and PepT2 are all members of proton-dependent oligopeptide transporters(POT) family. PepT1 is peptide transporter of low affinity and high capacity whereas PepT2 is high affinity and low capacity. PepT1 is mainly expressed in intestinal tract and weakly expressed in kidney, but PepT2 is mainly expressed in kidney. These transporters have some significant molecular characteristics: (1) Contain 12 membrane-spanning domains and a large extracellular loop between transmembrane domains 9 and 10, and all of the transmembrane domains are highly conserved, the extracellular loops are much less so; (2) The encoded proteins have a number of potential N-glycosylation as well as protein kinase recognition sites, which may be involved in the regulation of peptide transport; (3) His-57 in PepT1 and His-87 in PepT2 are the most critical histidyl residues that are necessary for the uptake function and probably represent some critical binding sites; (4) The proteins range in size from 707 to 729 amino acid residues in various species, with higher homology between species for a given transporter (approximately 80%) and less homology between transporters for a given species (approximately 50%). With the knowledge of the molecular characteristics and tissue distribution of these peptide transporters, we can understand the molecular mechanism underlying the peptide absorption, which may improve the peptide-drugs development in the future.