Salmo salar oligopeptide transporters PepT1a and PepT1b: a comparative electrophysiological characterization of partial and complete transport cycle

The H+-coupled peptide transporter 1 (PepT1) belongs to SoLute Carrier family 15 (SLC15A1) and is responsible for the absorption of di/tripeptides in enterocytes. Beside its nutritional role, it has been hypothesized that PepT1 functions as a transceptor, i.e. a peptide sensor/transporter involved in gut hormone release from entero-endocrine cell(s)1, 2. Studying the role of PepT1 in peptide absorption in the gut is relevant for the direct relation between dietary protein availability and fish growth3. In salmonids, PepT1 gene has been duplicated and two transporters, i.e. PepT1a and PepT1b, have been found in the intestine. The partial and complete transport cycle of the two PepT1-type transporters of Salmo salar, ssPepT1a and ssPepT1b, heterologously expressed in Xenopus laevis oocytes, were studied using Two Electrode Voltage Clamp technique. The pre-steady state currents of ssPepT1b were like that of other fish orthologs4 but differed from those of ssPepT1a. ssPepT1a showed a slower decaying currents, and the charge vs voltage (Q/V) and time constant vs voltage (τ/V) curves shifted to more positive potentials behaving as the mammalian transporter4. In both transporters, reducing external pH from 7.6 to 6.5 slowed the transients decay, shifting to more positive potential the Q/V and τ/V curves (Fig. 1). To evaluate the transport activity of ssPepT1a and ssPepT1b, the transport-associated currents were recorded in presence of 1mM of lysine(Lys)-containing peptides (KcPeps) as Lys is a limiting amino acid for animal growth3. KcPeps elicited transport-associated currents of different amplitudes, for e.g. ssPepT1b generated large currents when exposed to peptides carrying Lys in the N-terminus (KG, KM), while ssPepT1a produced small currents independently of Lys position (Fig. 2). The current vs voltage (I/V), in the presence of KcPeps, showed small and similar currents at two different pH (6.5, 7.6) in ssPepT1a. Conversely, in ssPepT1b the I/V curves differed at the most negative potentials with larger currents recorded at pH 7.6. KG dose-response experiments were also conducted, and while fitting with logistic equation allowed to obtain the kinetic parameters (K0.5 and Imax) at each voltage for ssPepT1b, only an estimation was possible for ssPepT1a at -140 and -120mV. The analysis on the transient and transport currents indicated important functional differences between ssPepT1a and ssPepT1b transporters. The dissimilar substrate specificity for KcPeps supports the idea of distinct roles in peptide recognition and transport for ssPepT1a and ssPepT1b.