ROLE OF THE C-TERMINAL TAIL OF THE GLUT1 GLUCOSE-TRANSPORTER IN ITS EXPRESSION AND FUNCTION IN XENOPUS-LAEVIS OOCYTES

Structural determinants for the glucose transport kinetics of the eryt hrocyte glucose transporter have not been established. in this work th e role of the cytosolic carboxy-terminal tail in the expression and fu nction of the human GLUT1 isoform in Xenopus oocytes was investigated. Oocyte plasma membrane expression of GLUT1 was a saturable function o f the amount of mRNA injected. Transport activity increased as a linea r function of the amount of immunoreactive transporter in the plasma m embrane. Transport kinetics of human GLUT1 expressed in oocytes resemb led those of human erythrocyte GLUT1. Addition of up to 31 extra amino acids to the carboxy-terminal tail of GLUT1 was without effect on its function in oocytes. Removal of the carboxy-terminal 21 amino acids a lso did not affect GLUT1 expression or transport kinetics in oocytes. Removal of the entire carboxy-terminal tail to Phe-450 resulted in a t ransporter that had moderately decreased plasma membrane expression co mpared to that of GLUT1. However, transport activity of this construct was less than 5% of that of GLUT1, and was associated with loss of it s outward-facing inhibitor binding site. When the carboxy-terminal 29 amino acids of GLUT1 were replaced with the corresponding region of GL UT4, transporter expression in the plasma membrane and the transport V -max fell to low levels, similar to those of native GLUT4. When the ca rboxy-terminal 29 or 73 amino acids of GLUT1 were swapped into the cor responding region of GLUT4, the transport V-max markedly increased to about one-third to one-half that of GLUT1, although the affinity for s ubstrate was halved. These results show that the carboxy-terminal tail of the GLUT1 is not critical for targeting of the protein to the plas ma membrane, but that this region is an important determinant of trans port function.