Oligomeric states of the insulin receptor: binding and autophosphorylation properties

Properties of oligomeric states of the insulin receptor were analyzed by polyacrylamide gel electrophoresis in nondenaturing buffer conditions (ND-PAGE). Partially purified insulin receptors resolve in ND-PAGE as three distinct species: (i) the fast electrophoretic mobility, low molecular mass form manifests intense labeling by iodinated insulin and shows basal and insulin-stimulated autophosphorylation; (ii) the middle, intermediate mobility form exhibits strong labeling by iodinated ligand but does not possess the capacity to be autophosphorylated; (iii) the slow mobility, highest molecular mass form necessitates covalent binding with iodinated hormone to withstand electrophoresis and shows autophosphorylation enhanced by insulin. This receptor form is more heavily labeled by phosphorylation than the low form. At 22 degrees C, binding and autophosphorylation do not appear to be time dependent. At 37 degrees C, binding and autophosphorylation of low and high species attain a maximum after 15 min and then decrease as time of incubation with insulin is prolonged to 120 min; the middle species exhibits a much slower association rate, and its labeling by iodinated hormone becomes more intense with time. Our data show that in cell-free systems insulin receptors appear in various oligomeric states and that the highest molecular mass oligomer exhibits the most pronounced autophosphorylation. This is compatible with the concept that insulin receptor oligomerization provides a mechanism for transmembrane signaling.