NPXY motif in the insulin-like growth factor-I receptor is required for efficient ligand-mediated receptor internalization and biological signaling.

The NPXY motif that was identified in the low density lipoprotein receptor serves as an internalization signal, and subsequent studies have indicated that the NPXY sequence is also an important recognition element for internalization of both insulin and transferrin receptors. The insulin-like growth factor (IGF-I) receptor contains an NPXY sequence (residues 947-950) in the immediate submembranous domain, and we sought to determine whether these residues play a role in facilitating ligand-mediated internalization of the IGF-I receptor. To study this, we have constructed stable cell lines expressing NPXY deletion mutant receptors (CHONPXY) or wild-type receptors (CHOWT). Ligand internalization studies showed that CHONPXY cells internalized [125I]IGF-I with a 40% defect (P < 0.005) compared to the CHOWT cells. Ligand-mediated receptor internalization and down-regulation were decreased by 90% in CHONPXY compared to CHOWT cells. The kinase activity of the mutant receptors was also defective as a result of removal of the NPXY motif. This is in contrast to the insulin receptor, in which deletion of the NPXY motif or deletions of even larger portions of amino acids in proximity to the NPXY motif do not affect the kinase activity of the receptor. Finally, the ability of the mutant receptors to mediate biological signaling is also defective, as measured by the thymidine incorporation assay.