Structural studies of the detergent-solubilized and vesicle-reconstituted insulin receptor

Insulin binding to the insulin receptor initiates a cascade of cellular eve nts that are responsible for regulating cell metabolism, proliferation, and growth. We have investigated the structure of the purified functionally ac tive, human insulin receptor using negative stain and cryo-electron microsc opy. Visualization of the detergent-solubilized and vesicle-reconstituted r eceptor shows the alpha(2)beta(2) heterotetrameric insulin receptor to be a three-armed pinwheel-like complex that exhibits considerable variability a mong individual receptors. The alpha-subunit of the receptor was labeled wi th an insulin analogue streptavidin gold conjugate, which facilitated the i dentification of the receptor arm responsible for insulin binding. The gold label was localized to the tip of a single receptor arm of the three-armed complex. The beta-subunit of the insulin receptor was labeled with a malei mide-gold conjugate, which allowed orientation of the receptor complex in t he membrane bilayer. The model derived from electron microscopic studies di splays a "Y"-like morphology representing the predominant species identifie d in the reconstituted receptor images. The insulin receptor dimensions are approximately 12.2 nm by 20.0 nm, extending 9.7 nm above the membrane surf ace. The beta-subunit-containing arm is approximately 13.9 nn, and each alp ha-subunit-containing arm is 8.6 nm in length. The model presented is the f irst description of the insulin receptor visualized in a fully hydrated sta te using cryo-electron microscopy.