J. Fischbarg et al., EVIDENCE THAT FACILITATIVE GLUCOSE TRANSPORTERS MAY FOLD AS BETA-BARRELS, Proceedings of the National Academy of Sciences of the United Statesof America, 90(24), 1993, pp. 11658-11662
A widely accepted model for the structure of the facilitative glucose
transporters (GLUTs) predicts that they form 12 transmembrane alpha-he
lices and that the highly conserved sequence Ile-386-Ala-405 in GLUT1
is intracellular. We raised a polyclonal antibody against a synthetic
peptide encompassing this conserved sequence and found that antibody t
reatment increased 2-deoxy-D-glucose (DOG) uptake in Xenopus oocytes e
xpressing GLUT1, GLUT2, or GLUT4 only when applied to the extracellula
r side. This effect was dose dependent and was specifically blocked by
competition with the peptide Ile-386-Ala-405; it was due to a decreas
e in the K(m) for the transport of DOG. To ascertain GLUT orientation,
we raised anti-peptide antibodies against the last 21 and 25 C-termin
al amino acids of GLUT1 and GLUT4, respectively, which were previously
shown to be intracellular. These antibodies increased DOG uptake when
injected into oocytes expressing GLUT1 and GLUT4, but not when added
extracellularly. Prompted by the noted discrepancy, we found sequence
similarity between GLUTs and porins, two of which are known from cryst
allography to form 16-stranded transmembrane antiparallel beta-barrels
. Analysis of the hydrophobicity, amphiphilicity, and turn propensity
of GLUT1 leads us to propose that GLUTs fold as porin-like transmembra
ne beta-barrels. This model is consistent with the results of the pres
ent antibody studies and also with previously published experimental e
vidence inconsistent with the 12-helix model.