Distinct hydrodynamic forms of the insulin receptor: electrophoretic analysis of the RI and RII species.

In previous work, we identified two insulin receptor species, RI (KAV = 0.31) and RII (KAV = 0.53), that could be separated by gel filtration on Sepharose 6B. In the present study, we sought to establish that these two receptor species do represent larger (RI) and smaller (RII) oligomeric forms of the receptor, rather than representing receptor species separated from each other by differential adsorption to the Sepharose matrix. Receptor solubilized from isolated human placenta membranes was purified by lectin- and insulin-agarose chromatography and was radiolabeled with carrier-free 125I. The labeled receptor was separated by Sepharose 6B gel filtration into two fractions (peak I, KAV = 0.31; peak II, KAV = 0.53), was immunoprecipitated by anti-insulin receptor antibody, and was analysed by electrophoresis in nonreducing polyacrylamide slab gels. The autoradiograms of the gels indicated that peak I (KAV = 0.31, RI receptor form) contained a number of receptor species of 240 000 daltons or greater, whereas peak II (KAV = 0.53, RII receptor form) contained mainly receptor species of 210 000 daltons or smaller. In particular, large amounts of a 90 000 dalton species (presumably free receptor beta-subunit) were present in peak II. Incubation of the material obtained from peak I with insulin resulted in a change in the electrophoretic pattern, which became identical with that observed for material recovered from peak II.(ABSTRACT TRUNCATED AT 250 WORDS)