Erythrocyte nucleoside and sugar transport Endo-β-galactosidase and endoglycosidase-F digestion of partially purified human and pig transporter proteins

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Abstract
Nucleoside- and glucose-transport proteins isolated from human erythrocyte membranes were photoaffinity-labelled with [3H]nitrobenzylthioinosine and [3H]cytochalasin B, respectively, and subjected to endo-.beta.-galactosidase or endoglycosidase-F digestion. Without enzyme treatment the two radiolabelled transporters migrated on SDS/polyacrylamide gels with the same apparent Mr (average) of 55000. Apparent Mr (average) values after endo-.beta.-galactosidase digestion were 47000 and 48000 for the nucleoside and glucose transporters respectively, and 44000 and 45000 respectively after endoglycosidase-F digestion. In contrast, endo-.beta.-galactosidase had no effect on the electrophoretic mobility of the nucleoside transporter isolated from pig erythrocytes. This transport system exhibited a higher Mr than the human protein, endoglycosidase-F treatment decreasing its apparent Mr (average) from 64000 to 57000. It is concluded that the human and pig erythrocyte nucleoside transporters are glycoproteins containing N-linked oligosaccharide. The data provide evidence of substantial carbohydrate and polypeptide differences between the human and pig erythrocyte nucleoside transporters, but evidence of molecular similarities between the human erythrocyte nucleoside and glucose transporters.

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