A Novel Insulin-Binding Protein with a Molecular Weight of 30, 000 Daltons

Analysis of mouse Swiss/3T3 fibroblasts and rat hepatoma H35 cells using the affinity cross-linking method revealed multiple forms of 125I-insulin binding components (Mr >300, 000) in the absence of reducing agents. The same analysis, in the presence of reducing agents, revealed two major com-ponents (Mr=125, 000 and Mr=30, 000). The Mr=125, 000 component ap-peared to be the α-subunit of the high-affinity insulin receptor, whereas the small insulin-binding component of Mr=30, 000 was not a degradation product of the a-subunit but was apparently associated with the insulin receptor. We suggest that it is likely a novel component for regulating the function of insulin receptor.