Chemical identity of the glucose transporter with the [ 3H]cytochalasin B-photolabelled component of human erythrocyte membranes. Equal sensitivity to trypsin and endoglycosidase F

Abstract

The protein photolabelled by [ 3H]cytochalasin B and band 4.5, which contains the human erythrocyte hexose transporter, were compared by electrophoretically monitoring the effect of digestion with endoglycosidase F and trypsin. Band 4.5 was found to consist of two minor components, Mr 58,000 and 52,000, and one main component, Mr 50,000–60,000. Deglycosylation by endoglycosidase F converted both the [ 3H]-labelled species and the main polypeptide of band 4.5 from a mixture of polypeptides of Mr 50,000–60,000 to a sharp component of Mr 46,000. Tryptic cleavage of the photolabelled protein produced a [ 3H]-labelled peptide of 19,000 daltons, which corresponded to an analogous tryptic fragment of the main component of band 4.5. Endoglycosidase F treatment of trypsin-treated samples had no effect on the 19,000 dalton fragment or the labelled 19,000 component, indicating that both species lack the carbonydrate moiety of the parent protein. This parallel chemical behaviour indicates that the photolabelled polypeptide is representative of the main constituent of band 4.5. Photolabelling may be used with confidence to quantitate glucose transporters in other cells.