The Role of Receptor Kinase Activity and the Npey⁹⁶⁰ Motif in Insulin-Accelerated Receptor-Mediated Insulin Internalization

Abstract

This study used biochemical and quantitative ultrastructural approaches to examine the roles that insulin receptor β subunit kinase activity, the NPEY motif in the juxtamembrane region, and tyrosine phosphorylation within that domain plays in insulin-accelerated receptor-mediated insulin internalization in CHO cells. Internalization of insulin in cells that expressed kinase-deficient receptors (CHO_A1018) or receptors lacking the NPEY Ala⁹⁵⁴-Asp⁹⁶⁵ domain (CHO_δ960) was reduced by 80% compared to cells expressing wild-type human insulin receptors (CHO_HIRc). Ultrastructural analysis revealed that the decreased internalization in CHO_A1018 cells was due to the reduced ability of the kinase deficient receptor to migrate from the microvilli of cultured cells and aggregate on the cell surface. Deletion of the NPEY motif in the juxtamembrane region of the β subunit severely reduced receptor migration, interfered with the normal aggregation of receptors on the cell surface, and virtually eliminated accumulation of the occupied receptors in the coated invaginations. Replacement of Tyr⁹⁶⁰ in the NPEY domain with phenylalanine (CHO_p960) had no significant effect on insulin internalization, receptor mobility, aggregation or accumulation in coated invaginations. In contrast, replacement of Tyr⁹⁶⁰ with alanine (CHO_A960) decreased insulin internalization, slowed migration, receptor aggregation and accumulation in coated invaginations. These studies document that kinase activity is required, but not sufficient, for receptor movement from the microvilli and aggregation of occupied receptors on the non-villous surface. An intact NPEY motif or surrounding amino acids, but not the phosphorylation of Tyr⁹⁶⁰ plays a role in receptor mobility and aggregation and is essential for the accumulation of insulin receptors in coated invaginations.