Differential phosphorylation of pp120 by insulin and insulin-like growth factor-1 receptors: role for the C-terminal domain of the beta-subunit.

pp 120, a plasma membrane glycoprotein expressed by hepatocytes, is a substrate of the insulin receptor tyrosine kinase. Since insulin-like growth factor-1 (IGF-1) and insulin receptors are structurally homologous, we investigated whether pp120 is also a substrate of the IGF-1 receptor tyrosine kinase. IGF-1 receptor failed to phosphorylate pp120 in response to IGF-1 in stably transfected NIH 3T3 fibroblasts. However, replacement of the C-terminal domain of the beta-subunit of the IGF-1 receptor with the corresponding fragment in the insulin receptor restored ligand-stimulated pp120 phosphorylation, suggesting that this domain plays a regulatory role in pp120 phosphorylation. Since pp120 is the first identified substrate specific for the insulin vis-à-vis the IGF-1 receptor tyrosine kinase, the pp120 signaling pathway may constitute a novel mechanism for the distinct cellular effects of insulin and IGF-1, the former being principally involved in metabolism, and the latter in mitogenesis.