upon insulin-treatment ofcells. SCAMPs were found ... GWG is a Lister Institute of. Preventive Medicine Research Fellow and DEI is a Wellcome Tmst. Research ...
Biochemical Society Transactions (1997) 25 4658 93
Characterisation of proteins associated with t h e GLUT4 intracellular compartment.
CAROLINE A. MILLAR*, DAVID E. JAMES# and GWYN W.
mu*.
*Division of Biochemistry and Molecular Biology, IBLS, University of Glasgow. Glasgow G12 8QQ. $Centre for Cellular and Molecular Biology, University of Queensland, St, Lucia, Brisbane. Qld 4072, Australia GLUT4, the glucose hansporter in muscle and adipose tissue, translocates from an intracellular site to the cell surface in response to insulin. enhancing the uptake of glucose into the cells (1,2). Previous studies have shown that endosomal proteins including the mannosed-phosphate receptor and the transfemn receptor also translocate to the plasma membrane in response to insulin uaderscoring the importance of this response to serve the general nutritional needs of the cell. However, in the case of insulinactivated glucose wansport, the fold increase of GLUT4 at the cell surface is much greater than the endosomal proteins suggesting the existence of a separate trafficking pathway and a distinct intmcllularcompartmenartment.To date the nabre of this intracellular compartment is poorly understaod. GLUT4 has been localised to tubulovesicular elements located in the tronr-Golgi reticulum, in cluste~ in~the cytoplasm near the plasma membrane and, at least in part, within the endocytic system as it undergoes constitutive recycling through clathrin-coated pits (3,4). Recent studies have identified several proteins shown to be associated with GLUT4 vesicles including; secretory carrier-associated membrane proteins (SCAMPS), vesicle-associated membrane protein (VAMP) and Raws). Also, a large proportion of intracellular GLUT4 and VAMP2 (synaptobrevin homologdvesicle-associated membrane protein) have been shown to be in a compartment distinct from recycling e n d m e s in non-stimulated adipocytes (5). As these proteins are ubiquitous to secretory vesicles and granules it is possible that GLUT4 may utilise the same mechanism of vesicle fusion as synaptic secretory vesicles (SSV) in nerve terminals (SNARE hypothesis), the non-endosomal pool of GLUT4 representing a secretory compartment. In this study we have characterised several proteins (syntaxin-4, y-adaptin. SCAMP,RIM, and the mannose-6-phosphate receptor) in 3T3-Ll adipoctyes and studied their intracellular distribution in response to both insulin stimulation and endosomal ablation with respect to GLUT4. To identify the cellular location of the aforementionedproteins and to determine their sensitivity to insulin, basal and insulin-treated 3T3-Ll adipocytes were subjected to subcellular fractionation followed by immunoblotting with antibodies specific for each protein as previously described (5). y-Adaptin and GLUT4 were found predominantly in the low density microsomal (LDM) fraction from basal cells. In response to insulin GLUT4 levels increased in the plasma membrane (PM) fraction showing a concomitant loss from the LDM fraction. y-Adaptin. a constituent of the AP2 complex involved in the assembly of clathrin coats of secretory vesicles at the tranr-Golgi reticulum. does not redistribute upon insulin-treatmentofcells. SCAMPs were found solely in the LDM fraction showing little redistribution upon insulin stimulation. SCAMPS (Secretory carrier associated Abbreviations -d membrane proteins), VAMP (Vesicle associated membrane protein; synaptobrevin). SSV (Synaptrc secretory vesicles), LDM (Low density microsomes), HDM (high density microsomes) PM (Plasma membrane) SP (Soluble proteins), Tf-HRP (TransfeninHorseradish Peroxidase).
Syntaxin 4, a type-2 membrane protein known to bind synaptobrevins and other proteins involved on secretory vesicle fusion was found predominantly in the PM f d o n with trace amounts in the LDM fraction, again showing little redistribution upon insulin stimulation. High levels of the mannose&phosphate recqtor. an endosumal p t e i n . were found in the LDM fraction from basal cells with a small increase in the PM fraction and an concomitant loss in the LDM fraction in response to insulin. Rab4. a small GTPbinding protein involved in regulated exocytosis. was located mainly in the LDM and HDM fractions with low levels in the PM and soluble protein (SP)fractions in basal cells. Upon insulin stimulationRab4 does not appear to redistribute in a similar manner to GLUT4. However, as Rab4 is not an integral membrane p t e i n and is thought to dissociate from GLUT4 vesicles in response to insulin. t r a n s l d a n to the PM may not be observed (6). In an attempt to further define the GLUT4 intracellular compartment we have examined the localisation of SCAMPs, yadaptin, the mannose-6-phmphate receptor and Rab4 in relation to GLUT4 with respect to endosomal ablation. Here we used Transfemn-Horseradish Peroxidase (Tf-HRP) cunjugate to ablate the recycling compartment in 3T3-Ll adipocytes. Total internal membranes were prepared and further subfractionated on discontinuous sucrose gradients as previously described (5). Fractions were collected from the gradients and analysed for the distribution of y-adaptin, SCAMPS, the mannose-6-phosphate receptor and Rab4. Our results from ablation analysis may be summarised as follows: *the mannose-6phosphate receptor appears to be completely ablated, in contrast to GLUT4 which is only partially ablated. *most of the y-adaptin is not ablated in these cells. *Rab4does not appear to be ablated in 3T3-Ll adipocytes. These data suggest that the non-endommal GLUT4 pool which is not ablated by Tf-HRP loading contains a significant proportion of y-adaptin and Raw. In contrast, the majority of the mannose-& phosphate receptor is ablated, and hence probably co-localises with GLUT4 in the endaomal recycling pool. We are grateful to Dr Margaret Robinson (University of Cambridge, Cambridge), Professor David Castle (University of Virginia, Charlottesville. Virginia) and Dr Peter van der Sluijs (Utrecht University School of Medicine,Utrecht) for the generous gift of reagents used in this study. This work was supported by The British Diabetic Association. GWG is a Lister Institute of Preventive Medicine Research Fellow and DEI is a Wellcome Tmst Research Fellow. CAM has a British Diabetic Association studentship. 1. Cushman, S. W. and Wardzala, L. J. (1980) J. Biol. Chem. 25 5,4758-4762 2. Suzuki, K.and Kono. T. (1980) Proc. Natl. A d . Sci. USA
7 7,2542-2545 3. Slot, J. W.. Geuze. H.J., Gigengack. S.. Lienhard, G.E. and James, D.E. (1991) J. Cell Biol. 113, 123-135 4. Robinson, L. J., and James, D. E. (1992) American Journal of Physiology 263.383-393 5. Livingstone, C., James, D. E.. Rice, J. E., Hanpeter, D. and a u l d , G. W. (1996) Biochem. J. 3 13 6. Cormont. M.. Tanti. J. F.. Zahmoui, A.. Van Obberghen. A., Tavatian, A. and Le Marchand-Brustel, Y.(1993) J. Biol. Chem. 268, 19491-19497
