DELETION OF 343 AMINO-ACIDS FROM THE CARBOXYL-TERMINUS OF THE BETA-SUBUNIT OF THE INSULIN-RECEPTOR INHIBITS INSULIN SIGNALING

Naturally occurring mutations in the insulin receptor gene that impair the receptor tyrosine kinase activity cause insulin resistance in viv o in a dominant fashion. Previously, two unrelated families have been described that express an insulin receptor with a truncation due to a premature chain termination at codon 1000 (Delta 1000), thereby deleti ng 343 amino acids from the carboxyl terminus of the beta-subunit. Whi le clinical findings suggest that the truncated receptor does not medi ate insulin action in vivo, a recent study suggested that a similarly truncated receptor enhanced insulin sensitivity in transfected cells b y augmenting the signaling by endogenous receptors [Sasaoka, T., Takat a, Y., Kusari, J., Anderson, C. M., Langlois, W. J., & Olefsky, J. M. (1993) Proc. Natl. Acad. Sci. U.S.A. 90, 4379-4383]. To investigate th ese paradoxical data, we studied the structure and function of Delta 1 000 truncated insulin receptors when expressed in NIH-3T3 cells. We fo und that, despite the deletion of most of the tyrosine kinase domain a nd all of the C-terminal domain of the beta-subunit of the insulin rec eptor, the Delta 1000 mutant receptors were processed normally and wer e transported to the plasma membrane where they bind insulin with high affinity. Following ligand addition, the truncated receptors are degr aded with a normal half-life. However, they fail to undergo insulin-st imulated internalization, do not regulate the phosphorylation of insul in receptor substrate 1, and are unable to mediate an insulin-stimulat ed increase in DNA synthesis and c-jun and c-fos expression. These res ults demonstrate that the Delta 1000 truncated receptors, expressed in our in vitro model system, faithfully mirror the in vivo findings tha t this mutation causes insulin resistance.