Facilitated glucose transporter of human erythrocyte: proteolytic mapping of the [3H]cytochalasin B photoaffinity-labeled transporter polypeptide

Myopia

Myopia, also known as short-sightedness or near-sightedness, is a very common condition that typically starts in childhood. Severe forms of myopia (pathologic myopia) are associated with a risk of other associated ophthalmic problems. This disorder affects all populations and is reaching epidemic proportions in East Asia, although there are differences in prevalence between countries. Myopia is caused by both environmental and genetic risk factors. A range of myopia management and control strategies are available that can treat this condition, but it is clear that understanding the factors involved in delaying myopia onset and slowing its progression will be key to reducing the rapid rise in its global prevalence. To achieve this goal, improved data collection using wearable technology, in combination with collection and assessment of data on demographic, genetic and environmental risk factors and with artificial intelligence are needed. Improved public health strategies focusing on early detection or prevention combined with additional effective therapeutic interventions to limit myopia progression are also needed.

Ligand-induced conformational changes modify proteolytic cleavage of the adipocyte insulin-sensitive glucose transporter

The transport conformation of the human erythrocyte glucose transporter (GLUT1) modifies rates of proteolytic cleavage of this protein by a variety of enzymes. We investigated the effects of ligand-induced conformational change on the susceptibility to enzymic cleavage of the insulin-sensitive rat adipocyte glucose transporter (GLUT4). A GLUT4-enriched slow sedimenting microsomal fraction was prepared from basal adipocytes and subjected to PAGE and immunoblotting. The GLUT4 protein was detected in these immunoblots with a C-terminal-specific antiserum as an M(r)-46,000-50,000 doublet. GLUT1 protein was not detected by a GLUT1-specific antiserum in these membranes. Tryptic digestion caused loss of the GLUT4 signal in immunoblots in a time- and concentration-dependent fashion. Low-M(r) membrane-bound fragments were not observed in electrophoretic gels, whether detection was attempted by immunoblotting or by counting radioactivity in gel slices following photolabelling with [3H]cytochalasin B. Transport-specific ligands known to induce an outward-facing conformation in the human erythrocyte GLUT1 protein retarded cleavage of the GLUT4 protein by submaximal concentrations of trypsin, whereas ligands known to induce an inward-facing conformation increased the extent of cleavage. The transported substrate D-glucose retarded tryptic cleavage of GLUT4. This result contrasts with the known behaviour of GLUT1, in which D-glucose accelerates cleavage. Cleavage of GLUT4 by thermolysin was also retarded by the outward-binding analogue 4,6-O-ethylidene glucose. These results show that the conformational sensitivity to proteolysis of GLUT4 mirrors that of GLUT1, except that the glucose-loaded GLUT4 has a different steady-state configuration, which may reflect underlying kinetic differences between the two proteins.

Localization of the forskolin photolabelling site within the monosaccharide transporter of human erythrocytes

Chemical and proteolytic digestion of intact erythrocyte glucose transporter as well as purified transporter protein has been used to localize the derivatization site for the photoaffinity agent 3-[125I]iodo-4-azido-phenethylamino-7-O-succinyldeacetylforskol in [( 125I]IAPS-forskolin). Comparison of the partial amino acid sequence of the labelled 18 kDa tryptic fragment with the known amino acid sequence for the HepG2 glucose transporter confirmed that the binding site for IAPS-forskolin is between the amino acid residues Glu254 and Tyr456. Digestion of intact glucose transporter with Pronase suggests that this site is within the membrane bilayer. Digestion of labelled transporter with CNBr generated a major radiolabelled fragment of Mr approximately 5800 putatively identified as residues 365-420. Isoelectric focusing of Staphylococcus aureus V8 proteinase-treated purified labelled tryptic fragment identified two peptides which likely correspond to amino acid residues 360-380 and 381-393. The common region for these radiolabelled peptides is the tenth putative transmembrane helix of the erythrocyte glucose transporter, comprising amino acid residues 369-389. Additional support for this conclusion comes from studies in which [125I]APS-forskolin was photoincorporated into the L-arabinose/H(+)-transport protein of Escherichia coli. Labelling of this transport protein was protected by both cytochalasin B and D-glucose. The region of the erythrocyte glucose transporter thought to be derivatized with IAPS-forskolin contains a tryptophan residue (Trp388) that is conserved in the sequence of the E. coli arabinose-transport protein.

Identification of the glucose transporter in mammalian cell membranes with a 125I-forskolin photoaffinity label

The glucose transporter has been identified in a variety of mammalian cell membranes using a photoactivatable carrier-free radioiodinated derivative of forskolin, 3-[125I]iodo-4-azidophenethylamido-7-O-succinyldeacetylforskoli n ([125I]IAPS-forskolin) at 1-3 nM. The membranes that were photolabelled with [125I]IAPS-forskolin were human placental membranes, rat cortical and cerebellar synaptic membranes, rat cardiac sarcolemmal membranes, rat adipocyte plasma membranes, smooth-muscle membranes, and S49 wild-type (WT) lymphoma-cell membranes. The glucose transporter in plasma membranes prepared from the insulin-responsive rat cardiac sarcolemmal cells, rat adipocytes and smooth-muscle cells were determined to be approx. 45 kDa by SDS/polyacrylamide-gel electrophoresis (PAGE). Photolysis of human placental membranes, rat cortical and cerebellar synaptic membranes, and WT lymphoma membranes with [125I]-IAPS-forskolin, followed by SDS/PAGE, indicated specific derivatization of a broad band (43-55 kDa) in placental membranes and a narrower band (approx. 45 kDa) in synaptic membranes and WT lymphoma membranes. Digestion of the [125I]IAPS-forskolin-labelled placental and WT lymphoma membranes with endo-beta-galactosidase showed a reduction in the apparent molecular mass of the radiolabelled band to approx. 40 kDa. The membranes that were photolabelled with [125I]IAPS-forskolin and trypsin-treated produced a radiolabelled proteolytic fragment with an apparent molecular mass of 18 kDa. [125I]IAPS-forskolin is a highly effective probe for identifying low levels of glucose transporters in mammalian tissues.

Photolabelling of the hexose transporter at external and internal sites: fragmentation patterns and evidence for a conformational change

The human erythrocyte sugar transporter has been labelled at its internal site with cytochalasin B and at its outside site by the azidosalicoyl derivative of bis(D-mannose) (ASA-BMPA). The cleavage of the transporter by various proteinases has been studied. Chymotrypsin, subtilisin and V8 proteinase give parallel fragmentation patterns for the two labels down to fragments as small as 7 kDa. Thus the binding sites for the two labels can only be separated by a small span of protein. 2-Nitro-5-thiocyanobenzoic acid (NTCB) cleaves at cysteines to give a 15 kDa fragment from the two labels. N-Bromosuccinimide (a reagent which preferentially cleaves at tryptophan residues) has revealed differences in fragmentation of the transporter labelled with either cytochalasin B or with ASA-BMPA. A major cleavage site is proposed to occur at tryptophan 186 which leaves a C-terminal fragment containing both labels. A tryptophan cleavage at residue 388 divides the cytochalasin B site and the ASA-BMPA site. A further tryptophan cleavage gives a cytochalasin B labelled 3 kDa fragment probably from residues 388-412. This gives an assignment of the cytochalasin B site as the inside of the hydrophobic span H 10. Since the ASA-BMPA site is probably only 7 kDa from residue 388 and is on the same 15 kDa NTCB fragment as cytochalasin B we assign this to the outside of hydrophobic span H 9. Thermolysin only cleaves the transporter labelled with cytochalasin B and not with ASA-BMPA. A 18 kDa cytochalasin B labelled fragment is formed. This is indicative of a change in conformation of the transporter when an outside ligand is bound such that the inside of the hydrogen bonding transmembrane segments H 7 and H 8 (and containing the proposed thermolysin cleavage site) are withdrawn from the cytosolic surface. Thus it appears that the core of the transporter (including the external and internal sites plus the transmembrane channel) is located between segments H 7 and H 10.