Single-molecule imaging of human insulin receptor ectodomain and its Fab complexes

The insulin receptor (IR) is a four-chain, transmembrane dimer held togethe r by disulfide bonds. To gain information about the molecular envelope and the organization of its domains, single-molecule images of the IR ectodomai n and its complexes with three Fabs have been analyzed by electron microsco py The data indicate that the IR ectodomain resembles a U-shaped prism of a pproximate dimensions 90 x 80 x 120 Angstrom. The width of the cleft (assum ed membrane-distal) between the two side arms is sufficient to accommodate ligand, Fab 83-7, which recognizes the cys-rich region of IR, bound halfway up one end of each side arm in a diametrically opposite manner, indicating a twofold axis of symmetry normal to the membrane surface. Fabs 83-14 and 18-44, which have been mapped respectively to the first fibronectin type II I domain (residues 469-592) and residues 765-770 in the insert domain, boun d near the base of the prism at opposite corners. These images, together wi th the data from the recently determined 3D structure of the first three do mains of the insulin-like growth factor type I receptor, suggest that the I R dimer is organized into two layers with the L1/cys-rich/L2 domains occupy ing the upper (membrane distal) region of the U-shaped prism and the fibron ectin type III domains and the insert domains located predominantly in the membrane-proximal region. (C) 1999 Academic Press.