The alpha beta monomer of the insulin receptor has hormone-responsive tyrosine kinase activity

Identification of the cysteine residues involved in the class I disulfide bonds of the human insulin receptor: properties of insulin receptor monomers

The cysteine residues involved in the class I disulfide bonds between the alpha subunits in the (alpha beta)2 dimer of the human insulin receptor have been identified by labeling with N-ethylmaleimide and by site-directed mutagenesis. Both cysteine 524 and cysteine 682 form interchain disulfide bonds; their conversion to serine residues results in the absence of receptor dimers and the presence of alpha beta monomers. The receptor monomers have a slightly lower affinity for insulin than the native receptor dimers. Insulin binding to the receptor monomers promotes their dimerization in the plasma membrane; at nanomolar concentrations of receptor, both unliganded and liganded receptors are monomers. Receptor monomers are stimulated by insulin to autophosphorylate and to phosphorylate exogenous subtrates with the same efficiency as the receptor dimers. The conclusion is that receptor dimerization is not required to activate the tyrosine kinase activity of the insulin receptor.

Organization of the transcortin-binding domain on placental plasma membranes

Complex formation between transcortin (corticosteroid-binding globulin) and 20 kDa sialoglycoprotein from human syncytiotrophoblast plasma membranes (presumably a transcortin-recognizing subunit of the transcortin membrane receptor) was studied using FPLC and cross-linking with bifunctional reagents. The action of 1,5-difluoro-2,4-dinitrobenzene (DFDNB) on a solution of the purified 20 kDa sialoglycoprotein and transcortin resulted in formation of covalently linked complexes of 95 kDa and 140 kDa consisting of one transcortin molecule and either two or four molecules of the membrane sialoglycoprotein (the molecular mass of transcortin is 55 kDa). Additionally, cross-linking resulted in the appearance of a 43 kDa species which is the cross-linked dimer of the membrane protein. The dimer was also observed during chromatography on a Superose 12 column in the absence of DFDNB treatment. Treatment of intact syncytiotrophoblast membranes with DFDNB resulted in isolation of the transcortin binding protein dimer as the major portion of total pool of the protein. Formation of the transcortin complexes with two and four molecules of the membrane protein was also observed when the membranes were incubated with 125I-labeled transcortin and treated with DFDNB, but formation of the latter complexes predominated. The results obtained suggest that the recognizing and binding domain for transcortin in placental membranes is organized as dimers consisting of non-covalently linked sialoglycoprotein monomers of a 20 kDa each and that transcortin has two sites for interaction with this dimer. Apparently, binding of two dimers results in the formation of the functional form of the transcortin-receptor complex. The possible biological role of such a complex is discussed.

Guanosine nucleotides regulate hormone binding of insulin receptors

Insulin receptors in turkey erythrocyte and rat adipocyte plasma membranes display non-linear hormone binding by Scatchard analysis. This result is consistent with evidence that the insulin-binding sites are heterogeneous and have at least two affinities for the hormone. Mild reduction of plasma membranes with dithiothreitol, before insulin binding, increased the fraction of hormone binding with high affinity without significantly changing the total number of receptor-binding sites. In the presence of guanosine 5'-[gamma-thio]triphosphate, the amount of receptor with high affinity for insulin in the reduced membranes decreased to that present in the absence of reduction; the effect of the nucleotide was concentration- and temperature-dependent. This decrease in insulin binding was specific for guanine nucleotides.