PARTIAL SEQUENCING AND TISSUTAL DISTRIBUTION OF A H+/PEPTIDE TRANSPORTER IN THE HAEMOGLOBINLESS ANTARCTIC TELEOST CHIONODRACO HAMATUS

The presence of a low affinity type H+/peptide transporter on the intestinal apical membranes of the Antarctic teleost Chionodraco hamatus, has been previously demonstrated (Ice-PepT; Maffia et al., J. Exp. Biol. 206: 705-714, 2003). The icefish peptide transporter even if kinetically resembled the mammalian PepT-1 isoform, exhibited specific cold adaptive features. In the present study the structural characteristics and the tissutal distribution of the transporter were investigated using RT-PCR and real-time RT-PCR analyses. An incomplete nucleotide sequence of 1744 pb was obtained, encoding for a protein region of 576 amino acid residues with an higher identity (57%) with hPepT1 in respect to hPepT2 (46%). The partial Ice-PepT amino acid sequence corresponded to nine of the twelve transmembrana (TMS) segments (II-X) reported in hPepT1, with a large extracellular loop between TMS IX and X, containing some potential N-linked glycosylation sites. The predicted protein also contained one potential site for protein kinase C-dependent phosphorylation in the putative intracellular loop between TMS X and XI (Ser 576) and one potential site for protein kinase A-dependent phosphorylation in the intracellular loop between TMS VIII and IX (Thr 330). The phylogenetic reconstruction among vertebrate H+/peptide transporters clustered Ice-PepT to the PepT1 branch of the phylogenetic tree and indicated early divergence of the fish protein sequence with respect to those of the tetrapod group. Ice-PepT mRNA was widely expressed in various tissues of C. hamatus, at highest levels in the intestinal mucosa and at a lesser extent in kidney, brain, liver, heart, and gills. In summary a novel member of a proton/oligopeptide transporter family has been partially characterised which can help to elucidate the evolutionary and functional relationship among H+/peptide transporters in vertebrates.