Replacement of intracellular C-terminal domain of GLUT1 glucose transporter with that of GLUT2 increases Vmax and Km of transport activity.

The intracellular C-terminal domain is diverse in size and amino acid sequence among facilitative glucose transporter isoforms. The characteristics of glucose transport are also divergent, and GLUT2 has far higher Km and Vmax values compared with GLUT1. To investigate the role of the intracellular C-terminal domain in glucose transport, we expressed in Chinese hamster ovary cells the mutated GLUT1 protein whose intracellular C-terminal domain was replaced with that of GLUT2 by means of engineering the chimeric cDNA. Cytochalasin B, for which GLUT2 protein has much lower affinity, bound to this chimeric protein in a fashion similar to GLUT1. In contrast, greater transport activity was observed in this chimeric glucose transporter compared with the wild-type GLUT1 at 10 mM 2-deoxy-D-glucose concentration. The kinetic studies on 2-deoxy-D-glucose uptake revealed a 3.8-fold increase in Km and a 4.3-fold increase in Vmax in this chimeric glucose transporter compared with the wild-type GLUT1. Thus, replacement of the intracellular C-terminal domain confers the GLUT2-like property on the glucose transporter. These results strongly suggest that the diversity of intracellular C-terminal domain contributes to the diversity of glucose transport characteristics among isoforms.     

Substitution of leucine for tryptophan 412 does not abolish cytochalasin B labeling but markedly decreases the intrinsic activity of GLUT1 glucose transporter

GLUT1 glucose transporter cDNA was modified to introduce a single amino acid substitution of leucine for tryptophan 412, a putative cytochalasin B photo-affinity labeling site. Although the mutated transporter was expressed into plasma membranes of Chinese hamster ovary cells, glucose transport activity of the mutated transporter was observed to be only 15-30% of that of the wild-type GLUT1 when glucose transport activity was assessed by 2-deoxyglucose uptake at 0.1-10 mM concentrations. Analysis of glucose uptake kinetics depict that a mutation induced a 3-fold decrease in turnover number and a 2.5-fold increase in Km compared with the wild-type GLUT1. Importantly, cytochalasin B labeling was not abolished but decreased by 40%, and cytochalasin B binding was also decreased. In addition, the results obtained with side-specific glucose analogs suggested that the outer glucose binding site of the mutant appeared intact but the inner binding site was modulated. These results indicate 1) tryptophan 412 is not a cytochalasin B labeling site(s), although this residue is located in or close to the inner glucose binding site of the GLUT1 glucose transporter, 2) substitution of leucine for tryptophan 412 decreases the intrinsic activity of GLUT1 glucose transporter, which is definable as the turnover number/Km, to approximately 15% of that of the wild-type.     

The role of N-glycosylation of GLUT1 for glucose transport activity.

To elucidate a functional role of N-glycosylation in glucose transporters, we introduced oligonucleotide-directed mutagenesis in GLUT1 cDNA to remove the possible site for N-linked glycosylation. The wild-type and the mutated GLUT1 cDNAs which induced a mutation of Asn at residue 45 to Asp, Tyr, or Gln were transfected and stably expressed into Chinese hamster ovary cells. The expressed wild-type and the mutated GLUT1 was demonstrated to be a broad band of a 45-60-kDa form and a sharp band of a 38-kDa form on Western blot analysis, respectively, indicating no glycosylation in the mutated GLUT1. Although the cell surface labeling of the glucose transporters demonstrated the presence of the glycosylation-defective glucose transporters on the cells surface, photoaffinity labeling of glycosylation-defective GLUT1 with [3H] cytochalasin B and a photoreactive mannose derivative, [3H]2-N-4-(1-azi-2,2,2,trifluoroethyl)benzoyl-1,3-bis(D-mannos+ ++-4-yloxy)-2- propylamine in the membranes was observed to be 40-70 and 15-30% of that of the wild-type GLUT1, respectively. The kinetic study of 2-deoxyglucose uptake revealed that the glycosylation-defective GLUT1 had a 2-2.5-fold greater Km value for 2-deoxyglucose uptake compared with the wild-type GLUT1. These observations strongly suggest that 1) N-glycosylation of GLUT1 glucose transporter is only on Asn 45 and 2) N-glycosylation plays an important role in maintaining a structure of glucose transporter with high affinity for glucose, thus, with high transport activity.     

Induction of a mutant phenotype in human repair proficient cells after overexpression of a mutated human DNA repair gene.

Antisense and mutated cDNA of the human excision repair gene ERCC-1 were overexpressed in repair proficient HeLa cells by means of an Epstein-Barr-virus derived cDNA expression vector. Whereas antisense RNA did not influence the survival of the transfected cells, a mutated cDNA generating an ERCC-1 protein with two extra amino acids in a conserved region of its C-terminal part resulted in a significant sensitization of the HeLa transfectants to mitomycin C-induced damage. These results suggest that overexpression of the mutated ERCC-1 protein interferes with proper functioning of the excision repair pathway in repair proficient cells and is compatible with a model in which the mutated ERCC-1 protein competes with the wild-type polypeptide for a specific step in the repair process or for occupation of a site in a repair complex. Apparently, this effect is more pronounced for mitomycin C induced crosslink repair than for UV-induced DNA damage.     

KCC2 transport activity requires the highly conserved L₆₇₅ in the C-terminal β1 strand

The activity of the neuron-specific K(+), Cl(-) co-transporter 2 (KCC2) is required for hyperpolarizing action of GABA and glycine. KCC2-mediated transport therefore plays a pivotal role in neuronal inhibition. Few analyses have addressed the amino acid requirements for transport-competent conformation. KCC2 consists of 12 transmembrane domains flanked by two intracellular termini. Structural analyses of a related archaeal protein have identified an evolutionary extremely conserved β1 strand, which links the transmembrane domain to a C-terminal dimerization interface. Here, we focused on the sequence requirement of this linker. We mutated four highly conserved amino acids of the β1 strand ((673)QLLV(676)) to alanine and analyzed the functional consequences in mammalian cells. Flux measurements demonstrated that L(675A) significantly reduced KCC2 transport activity by 41%, whereas the other three mutants displayed normal activity. Immunocytochemistry and cell surface labeling revealed normal trafficking of all four mutants. Altogether, our results identify L(675) as a critical residue for KCC2 transport activity. Furthermore, in view of its evolutionary conservation, the data suggest a remarkable tolerance of the KCC2 transport activity to amino acid substitutions in the β1 strand.     

Structural basis of GLUT1 inhibition by cytoplasmic ATP

Cytoplasmic ATP inhibits human erythrocyte glucose transport protein (GLUT1)-mediated glucose transport in human red blood cells by reducing net glucose transport but not exchange glucose transport (Cloherty, E.K., D.L. Diamond, K.S. Heard, and A. Carruthers. 1996. Biochemistry. 35:13231-13239). We investigated the mechanism of ATP regulation of GLUT1 by identifying GLUT1 domains that undergo significant conformational change upon GLUT1-ATP interaction. ATP (but not GTP) protects GLUT1 against tryptic digestion. Immunoblot analysis indicates that ATP protection extends across multiple GLUT1 domains. Peptide-directed antibody binding to full-length GLUT1 is reduced by ATP at two specific locations: exofacial loop 7-8 and the cytoplasmic C terminus. C-terminal antibody binding to wild-type GLUT1 expressed in HEK cells is inhibited by ATP but binding of the same antibody to a GLUT1-GLUT4 chimera in which loop 6-7 of GLUT1 is substituted with loop 6-7 of GLUT4 is unaffected. ATP reduces GLUT1 lysine covalent modification by sulfo-NHS-LC-biotin by 40%. AMP is without effect on lysine accessibility but antagonizes ATP inhibition of lysine modification. Tandem electrospray ionization mass spectrometry analysis indicates that ATP reduces covalent modification of lysine residues 245, 255, 256, and 477, whereas labeling at lysine residues 225, 229, and 230 is unchanged. Exogenous, intracellular GLUT1 C-terminal peptide mimics ATP modulation of transport whereas C-terminal peptide-directed IgGs inhibit ATP modulation of glucose transport. These findings suggest that transport regulation involves ATP-dependent conformational changes in (or interactions between) the GLUT1 C terminus and the C-terminal half of GLUT1 cytoplasmic loop 6-7.     

The glucose transport activity of GLUT1 is markedly decreased by substitution of a single amino acid with a different charge at residue 415

GLUT1 glucose transporter cDNA was modified to introduce a single amino acid substitution of aspartic acid for asparagine 415, which is conserved among all facilitative glucose transporter isoforms. Although a significant amount of the mutated transporter was expressed into plasma membranes of Chinese hamster ovary cells by transfection with expression vector, almost no increase in glucose transport activity was observed. Analysis of glucose uptake with Lineweaver-Burk plot depicts that the mutation induced a marked decrease (more than 5-fold) in turnover number and a slight increase (1.5-fold) in Km compared with the wild-type GLUT1. Results obtained with cytochalasin B and ethylidene glucose suggested that the inner but not outer glucose binding site was modulated. These results suggest that asparagine 415 is located close to the inner glucose binding site and the putative inner gate of GLUT1 glucose transporter and that an ionic charge in this domain might play an important role in the rate of conformational change between an inward-facing form and an outward-facing form of glucose transporter.     

Site-directed mutagenesis of GLUT1 in helix 7 residue 282 results in perturbation of exofacial ligand binding.

The structure-function relationship of the HepG2/erythrocyte-type glucose transporter (GLUT1) has been studied by in vitro site-directed mutagenesis. Chinese hamster ovary clones in which glucose transporters were transfected were shown by Western blotting with a GLUT1 anti-COOH-terminal peptide antibody to have expression levels of Gln282----Leu, Asn288----Ile, and Asn317----Ile mutations that were comparable with the wild type. All three mutant GLUT1 clones had high 2-deoxy-D-glucose transport activity compared with a nontransfected clone, suggesting that these residues are not absolutely required for the transport function. We have examined the possibility that the inner and outer portions of the transport pathway are structurally separate by measuring the interaction of the mutant transporters with the inside site-specific ligand cytochalasin B and the outside site-specific ligand 2-N-4-(1-azi-2,2,2-trifluoroethyl)benzoyl-1,3-bis(D-mannos-4 -yloxy)-2- propylamine (ATB-BMPA). All three mutant GLUT1 clones showed high levels of cytochalasin B labeling, and the N288I and N317I mutants showed high levels of ATB-BMPA labeling. In contrast to the transport and cytochalasin B labeling results, the transmembrane helix 7 Gln282----Leu mutant was labeled by ATB-BMPA to a level that was only 5% of the level observed in the wild type. We have confirmed that this mutant was defective in the outer site by comparing the inhibition of wild-type and mutant 2-deoxy-D-glucose transport by the outside site-specific ligand 4,6-O-ethylidene-D-glucose. 4,6-O-Ethylidene-D-glucose inhibited wild-type transport with a Ki of approximately 12 mM, but this was increased to greater than 120 mM in the Gln282----Leu mutant. Thus, of the 3 residues mutated in this study, only glutamine 282 substitution causes a major perturbation in function, and this is a specific and striking reduction in the affinity for the outside site-specific ligands ATB-BMPA and 4,6-O-ethylidene-D-glucose.     

Glycoprotein cytoplasmic domain sequences required for rescue of a vesicular stomatitis virus glycoprotein mutant.

We have used transient expression of the wild-type vesicular stomatitis virus (VSV) glycoprotein (G protein) from cloned cDNA to rescue a temperature-sensitive G protein mutant of VSV in cells at the nonpermissive temperature. Using cDNAs encoding G proteins with deletions in the normal 29-amino-acid cytoplasmic domain, we determined that the presence of either the membrane-proximal 9 amino acids or the membrane-distal 12 amino acids was sufficient for rescue of the temperature-sensitive mutant. G proteins with cytoplasmic domains derived from other cellular or viral G proteins did not rescue the mutant, nor did G proteins with one or three amino acids of the normal cytoplasmic domain. Rescue correlated directly with the ability of the G proteins to be incorporated into virus particles. This was shown by analysis of radiolabeled particles separated on sucrose gradients as well as by electron microscopy of rescued virus after immunogold labeling. Quantitation of surface expression showed that all of the mutated G proteins were expressed less efficiently on the cell surface than was wild-type G protein. However, we were able to correct for differences in rescue efficiency resulting from differences in the level of surface expression by reducing wild-type G protein expression to levels equivalent to those observed for the mutated G proteins. Our results provide evidence that at least a portion of the cytoplasmic domain is required for efficient assembly of the VSV G protein into virions during virus budding.     

Glucose transporter asymmetries in the bovine blood-brain barrier

The transport of glucose across the mammalian blood-brain barrier is mediated by the GLUT1 glucose transporter, which is concentrated in the endothelial cells of the cerebral microvessels. Several studies supported an asymmetric distribution of GLUT1 protein between the luminal and abluminal membranes (1:4) with a significant proportion of intracellular transporters. In this study we investigated the activity and concentration of GLUT1 in isolated luminal and abluminal membrane fractions of bovine brain endothelial cells. Glucose transport activity and glucose transporter concentration, as determined by cytochalasin B binding, were 2-fold greater in the luminal than in the abluminal membranes. In contrast, Western blot analysis using a rabbit polyclonal antibody raised against the C-terminal 20 amino acids of GLUT1 indicated a 1:5 luminal:abluminal distribution. Western blot analysis with antibodies raised against either the intracellular loop of GLUT1 or the purified erythrocyte protein exhibited luminal:abluminal ratios of 1:1. A similar ratio was observed when the luminal and abluminal fractions were exposed to the 2-N-4[(3)H](1-azi-2,2,2,-trifluoroethyl)benzoxyl-1,3-bis-(d-mannos-4-yloxyl)-2-propylamine ([(3)H]ATB-BMPA) photoaffinity label. These observations suggest that either an additional glucose transporter isoform is present in the luminal membrane of the bovine blood-brain barrier or the C-terminal epitope of GLUT1 is "masked" in the luminal membrane but not in the abluminal membranes.     

Mitochondrial processing of bovine adrenal steroidogenic acute regulatory protein

Steroidogenic acute regulatory (StAR) protein is an important regulatory protein in steroidogenesis and rapidly undergoes proteolysis after import into the mitochondria. In this study, we determined the proteolytic cleavage sites and investigated the effects on the stimulation of steroidogenic activity of the blockage of these sites by mutation. The cleaved StAR proteins, which were purified using an anti-StAR immobilized column, reacted with antiserum against the StAR C-terminal oligopeptide. The molecular weights of the purified proteins were determined by MALDI-TOF mass spectrometry, and were found to be identical to those of the 40-285 and 55-285 amino-acid-regions of the StAR protein. To confirm the identification of the cleavage sites, we constructed site-directed mutants of bovine StAR cDNA, which contained the amino acids R37A/R38A/L40A and/or R53A/R54A/R55A. These mutant StAR proteins expressed in COS-1 cells were not cleaved at positions 39-40 and 54-55, and were processed at sites different from those in the wild-type StAR protein. These mutant proteins stimulated pregnenolone formation at almost the same rate as the wild-type StAR protein in COS-1 cells, which suggests that the cholesterol transfer activity was not affected by the mutation.     

Monosaccharide uptake in common carp (Cyprinus carpio) EPC cells is mediated by a facilitative glucose carrier

In most animal cells, transport of monosaccharides across the plasma membrane is mediated by glucose transporters (GLUT). Mammals express at least five distinct transporters (GLUTs 1--5), which are well characterised both functionally and genetically. In contrast, the glucose transport system of fish remains poorly studied. Here we report studies of hexose uptake in carp EPC cells and cloning of a glucose transporter cDNA from these cells. Transport of radio-labelled methylglucose (3-OMG) followed Michaelis--Menten kinetics with a K(m) value (8.5 mM) similar to that of mammalian cells. The inhibition of transport by cytochalasin B and phloretin, but not by phloridzin or cyanide, strongly suggested the existence of a facilitative carrier. D-Glucose, 2-deoxyglucose, 3-OMG, D-mannose and D-xylose were competitive inhibitors of 3-OMG uptake, while L-glucose, mannitol, D-fructose, D-ribose and sucrose did not compete with 3-OMG. We cloned a carp glucose transporter (CyiGLUT1), using RT-PCR and RACE strategies. CyiGLUT1 was different from known carp and zebrafish EST sequences. The complete cDNA (3060 bp) contained one open reading frame encoding a predicted protein of 478 amino acids. The deduced amino acid sequence shared 78% identity with mammalian and avian GLUT1 proteins. Key amino acids involved in substrate selection and catalysis of mammalian GLUTs were conserved in the carp transporter.     

The posttranslationally modified C-terminal structure of bovine aortic smooth muscle rhoA p21

rhoA p21, a ras p21-like small GTP-binding protein, has the same C-terminal consensus motif of Cys-A-A-X (A is an aliphatic amino acid and X is any amino acid) as ras p21s, which is posttranslationally processed. We here determine the posttranslationally processed C-terminal structure of the rhoA p21 purified from bovine aortic smooth muscle. Incubation of rhoA p21-expressing insect cells with exogenous [3H]mevalonolactone caused the labeling of rhoA p21, suggesting that rhoA p21 is prenylated. Consistently, Raney nickel treatment of rhoA p21 released a geranylgeranyl moiety as estimated by gas chromatography/mass spectrometry. No lipid moiety was released by KOH or NH2OH treatment. Extensive digestion of rhoA p21 with Achromobacter protease I yielded a C-terminal peptide, Ser-Gly-Cys190, that lacked the three C-terminal amino acids predicted from the cDNA but was geranylgeranylated and carboxyl methylated at the cysteine residue. Bovine brain cytosol geranylgeranylated the bacterial rhoA p21 having the three C-terminal amino acids predicted from the cDNA but not the protein lacking the three C-terminal amino acids. Bovine brain membranes methylated the synthetic C-terminal peptide with 10 amino acids of rhoA p21 which was geranylgeranylated at its C-terminal cysteine residue but not the peptide which was not geranylgeranylated. These results suggest that rhoA p21 is first geranylgeranylated followed by removal of the three C-terminal amino acids and the subsequent carboxyl methylation of the exposed cysteine residue.     

GLUTX1, a novel mammalian glucose transporter expressed in the central nervous system and insulin-sensitive tissues

Based on homology with GLUT1-5, we have isolated a cDNA for a novel glucose transporter, GLUTX1. This cDNA encodes a protein of 478 amino acids that shows between 29 and 32% identity with rat GLUT1-5 and 32-36% identity with plant and bacterial hexose transporters. Unlike GLUT1-5, GLUTX1 has a short extracellular loop between transmembrane domain (TM) 1 and TM2 and a long extracellular loop between TM9 and TM10 that contains the only N-glycosylation site. When expressed in Xenopus oocytes, GLUTX1 showed strong transport activity only after suppression of a dileucine internalization motif present in the amino-terminal region. Transport activity was inhibited by cytochalasin B and partly competed by D-fructose and D-galactose. The Michaelis-Menten constant for glucose was approximately 2 mM. When translated in reticulocytes lysates, GLUTX1 migrates as a 35-kDa protein that becomes glycosylated in the presence of microsomal membranes. Western blot analysis of GLUTX1 transiently expressed in HEK293T cells revealed a diffuse band with a molecular mass of 37-50 kDa that could be converted to a approximately 35-kDa polypeptide following enzymatic deglycosylation. Immunofluorescence microscopy detection of GLUTX1 transfected into HEK293T cells showed an intracellular staining. Mutation of the dileucine internalization motif induced expression of GLUTX1 at the cell surface. GLUTX1 mRNA was detected in testis, hypothalamus, cerebellum, brainstem, hippocampus, and adrenal gland. We hypothesize that, in a similar fashion to GLUT4, in vivo cell surface expression of GLUTX1 may be inducible by a hormonal or other stimulus.     

Fusion formation by the uncleaved spike protein of murine coronavirus JHMV variant cl-2.

The fusogenic properties of the uncleaved spike (S) protein of murine coronavirus JHMV variant cl-2 were studied by expressing the S protein with a deleted putative cleavage site. The amino acid sequence of the putative cleavage site, Arg-Arg-Ala-Arg-Arg, was replaced by Arg-Thr-Ala-Leu-Glu by in vitro mutagenesis of the cl-2 S protein cDNA. Recombinant vaccinia viruses containing the cl-2 S cDNA [RVV t(+)] or the mutated cDNA [RVV t(-)] were constructed and monitored for fusion formation and cleavage of the expressed S proteins. When cultured DBT cells were infected with RVV t(+) at a multiplicity of infection of 0.5, fusion formation was first observed at 10 to 12 h postinoculation and spread throughout the whole culture by 20 to 24 h postinoculation. In cells infected with RVV t(-) under the same conditions, fusion formation appeared by 12 to 14 h. This result represented a 2- to 4-h delay in the onset of fusion, compared with its appearance in cells expressing the wild-type S protein. By 25 to 30 h, most of the cells infected by RVV t(-) had fused. By immunoprecipitation and Western blotting (immunoblotting), the 170-kDa S protein was detected in DBT cells expressing the wild-type S protein and the mutated S protein. However, interestingly, the cleavage products of the S protein, S1 and S2, were not detected in RVV t(-)-infected cells, producing the mutated S protein, even though fusion was clearly visible. Both products were, of course, detected in RVV t(+)-infected DBT cells, producing the wild-type S protein. The same results concerning the fusion formation and cleavage properties of the S proteins were reproduced by the transiently expressed S proteins. These results suggest that the cleavage event in the S protein of murine coronavirus JHMV is not a prerequisite for fusion formation but that it does facilitate fusion formation.     

Detection of insulin-regulated GLUT4-translocation by the insertion of a protein C epitope in L6 myoblasts

Insulin stimulates glucose transport by translocation of the membrane glucose transporter GLUT4 from intracellular vesicles to the plasma membrane. GLUT4 is highly expressed in adipose tissue and skeletal muscle. We have constructed a cDNA containing the human GLUT4 inserted by a 12 amino acid protein C epitope in the first extracellular (exofacial) domain of the human GLUT4 (GLUT4-PC). Stable expression of GLUT4-PC in L6 myoblasts (L6-GLUT4-PC) was confirmed in immunofluorescence using monoclonal antibodies against protein C. The protein C staining yielded labeling in perinuclear vesicles strongly co-localizing with GLUT4 detected with antibodies directed against the endofacial part of GLUT4. The L6-GLUT4-PC cells were further characterized in a direct cell-based enzyme-linked immunosorbent assay by the use of beta-galactosidase. Cell surface binding of monoclonal protein C antibodies was detected with beta-galactosidase-conjugated secondary antibodies and chlorophenolred-beta-D-galactopyranoside (CPRG) as substrate in 2% paraformaldehyde fixed cells. In this assay, stimulation with insulin created a rapidly detectable recruitment of GLUT4-PC to the cell surface. This cell-based enzyme-linked immunosorbent GLUT4 assay was shown to be comparable with that of previously reported radioactive assays.     

The C-terminal tetrapeptide HWFW of the Chlorella HUP1 hexose/H(+)-symporter is essential for full activity and an alpha-helical structure of the C-terminus

C-terminal tails of plant hexose/H(+)-symporters of the major facilitator superfamily contain a highly conserved motif of four amino acids: HWFW. A deletion of these four amino acids in the Chlorella HUP1 protein leads to a decrease in transport activity by a factor of 3-4. The mutated tail is highly sensitive to trypsin; it does not show alpha-helical conformation in contrast to the wild type C-terminal peptide with an alpha-helical content of at least 15%. The production of monoclonal antibody 416B8 recognizing an epitope within the central loop of HUP1 protein has been a prerequisite for the experiments described.     

Expression of the glucose transporter isoform GLUT 4 is insufficient to confer insulin-regulatable hexose uptake to cultured muscle cells.

Glucose transporter isoform expression was studied in the skeletal muscle-like cell line, C2C12. Northern and Western blot analysis showed that the insulin-responsive muscle/fat glucose transporter isoform, GLUT 4, was expressed in these cells at very low levels, whereas the erythrocyte isoform, GLUT 1, was expressed at readily detectable levels. Insulin did not stimulate glucose transport in this cultured muscle cell line. The C2C12 cells were then transfected separately with either GLUT 1 or GLUT 4, and stable cell lines expressing high levels of mRNA and protein were isolated. GLUT 1-transfected cells exhibited a 3-fold increase in the amount of the GLUT 1 transporter protein which was accompanied by a 2- to 3-fold increase in the glucose uptake rate. However, despite at least a 10-fold increase in GLUT 4 mRNA and protein detected after GLUT 4 cDNA transfection, the glucose uptake of these cells was unchanged and remained insulin-insensitive. By laser confocal immunofluorescence imaging, it was established that the transfected GLUT 4 protein was localized almost entirely in cytoplasmic compartments. In contrast, the GLUT 1 isoform was detected both at the plasma membrane as well as in intracellular compartments. These results suggest that acute insulin stimulation of glucose transport is not solely dependent on the presence of the insulin receptor and the GLUT 4 protein, and that the presence of some additional protein(s) must be required.