Identification of a candidate membrane protein for the basolateral peptide transporter of rat small intestine.

A candidate protein for the basolateral peptide transporter of rat jejunum is described. Vascular perfusion of the photoaffinity label, [4-azido-D-phe]-L-ala (2.5 mM), had no effect on the transepithelial transport of the non-hydrolysable dipeptide D-phe-L-gln (1 mM) from the lumen, its mucosal accumulation or wash-out into the vascular perfusate. When the label was perfused luminally, the transepithelial transport of D-phe-L-gln was inhibited by 38% ðP < 0:001Þ and accumulation increased by 62% ðP < 0:05Þ. These data are consistent with those of a basolateral transporter that is strongly asymmetric in its substrate binding and transport properties. Labelling of basolateral membrane vesicles with [4-azido-3,5- 3 H-D-phe]-L-ala revealed that the majority of label was incorporated into a single protein of Mr 112 � 2 kDa and p I6 :5. MALDI-TOF analysis of tryptic digests of the protein followed by database searches established that this protein was novel with no obvious similarity to PepT1, the apical membrane transporter. 2002 Elsevier Science (USA). All rights reserved.