Characterization of the Met326Ile variant of phosphatidylinositol 3-kinase p85α

Phosphatidylinositol 3-kinase is a key step in the metabolic actions of insulin. Two amino acid substitutions have been identified in the gene for the regulatory subunit of human p85α, Met-326Ile, and Asn-330Asp, and the former has been associated with alterations in glucose/insulin homeostasis. When the four human p85α proteins were expressed in yeast, a 27% decrease occurred in the level of protein expression of p85α(Ile/Asp) (P = 0.03) and a 43% decrease in p85α(Ile/Asn) (P = 0.08) as compared with p85α(Met/Asp). Both p85α(Ile/Asp) and p85α(Ile/Asn) also exhibited increased binding to phospho-insulin receptor substrate-1 by 41% and 83%, respectively (P < 0.001), as compared with p85α(Met/Asp). The expression of p85α(Ile) was also slightly decreased and the binding to insulin receptor substrate-1 slightly increased in brown preadipocytes derived from p85α knockout mice. Both p85α(Met) and p85α(Ile) had similar effects on AKT activity and were able to reconstitute differentiation of the preadipocytes, although the triglyceride concentration in fully differentiated adipocytes and insulin-stimulated 2-deoxyglucose uptake were slightly lower than in adipocytes expressing p85α(Met). Thus, the Met-326Ile variant of p85α is functional for intracellular signaling and adipocyte differentiation but has small alterations in protein expression and activity that could play a role in modifying insulin action.