Chromatographic characterization of nitrobenzylthioinosine binding proteins in band 4.5 of human erythrocytes: purification of a 40 kDa truncated nucleoside transporter

Abstract

DEAE-column-purified band 4.5 polypeptides of human erythrocyte membranes are mostly glucose transporters with nucleoside transporters as a minor component. The purpose of the present work was to differentially identify and isolate the nucleoside transporters in band 4.5 free from glucose transporters. Equilibrium binding studies demonstrated that the band 4.5 preparation binds nibrobenzylthioinosine (NBTI), a potent nucleoside transport inhibitor, at two distinct sites, one with a high affinity (dissociation constant, KD of 1 nM) with a small capacity, BT (0.4 nmol/mg protein), and the other with a low affinity (KD of 15 microM) with a large BT (14-16 nmol/mg protein). The BT of the low-affinity site was equal to that of the cytochalasin B binding site in the preparation. A gel-filtration chromatography of band 4.5 photolabeled with [3H]NBTI and [3H]cytochalasin B identified three polypeptides of apparent Mr 55,000, 50,000 and 40,000. Of these, the 55 kDa polypeptide was specifically labeled by cytochalasin B (p55GT), indicating that it is a glucose transporter. Both the 50 and 40 kDa polypeptides were labeled with NBTI at low ligand concentrations (less than 0.1 microM), which was abolished by an excess (20 microM) of nitrobenzylthioguanosine, indicating that they are two forms (p50NT and p40NT, respectively) of the high affinity NBTI binding protein or nucleoside transporter. At higher (not less than 10 microM) NBTI concentrations, however, p55GT was also labeled with NBTI, indicating that the low-affinity NBTI binding is due to a glucose transporter. Treatment of band 4.5 with trypsin reduced the p50NT labeling with a concomitant and stoichiometric increase in the p40NT NBTI labeling without affecting the high-affinity NBTI binding of the preparation. These findings indicate that the nucleoside transporter is slightly smaller in mass than the glucose transporter and that trypsin digestion produces a truncated nucleoside transporter of apparent Mr 40,000 which retains the high-affinity NBTI binding activity of intact nucleoside transporter. Both p55GT and p50 NT were coeluted in a major protein fraction, P1 in the chromatography, while p40NT was eluted separately as a minor protein fraction, P1a. All three polypeptides formed mixed dimers, which were eluted in a fraction PO. We have purified and partially characterized the truncated nucleoside transporter, p40NT. The purified p40NT may be useful for biochemical characterization of the nucleoside transporter.