All affected patients in four families with autosomal dominant familial renal tubular acidosis (dRTA) were heterozygous for mutations in their red cell HCO 3 Ϫ /Cl Ϫ exchanger, band 3 ( AE1, SLC4A1 ) genes, and these mutations were not found in any of the nine normal family members studied. The mutation Arg 589 → His was present in two families, while Arg 589 → Cys and Ser 613 → Phe changes were found in the other families. Linkage studies confirmed the co-segregation of the disease with a genetic marker close to AE1 . The affected individuals with the Arg 589 mutations had reduced red cell sulfate transport and altered glycosylation of the red cell band 3 N-glycan chain. The red cells of individuals with the Ser 613 → Phe mutation had markedly increased red cell sulfate transport but almost normal red cell iodide transport. The erythroid and kidney isoforms of the mutant band 3 proteins were expressed in Xenopus oocytes and all showed significant chloride transport activity. We conclude that dominantly inherited dRTA is associated with mutations in band 3; but both the disease and its autosomal dominant inheritance are not related simply to the anion transport activity of the mutant proteins. ( J. Clin. Invest. 1997.

The physiological roles of insulin and insulin-like growth factors (IGFs) are distinct, with insulin acting to regulate cellular uptake and metabolism of fuels, whereas IGFs promote cell growth, survival and differentiation. The only components of signalling pathways known to be unique to insulin and IGFs are their respective receptors, and even these display substantial structural and functional similarity. Specificity of action in vivo must in part reflect relative levels of receptor expression in different tissues. The extent to which the receptors differ in intrinsic signalling capacity remains unclear, but specificity might in principle arise from differences in ligand-binding mechanism or properties of intracellular domains. To identify ligand binding determinants we expressed receptor fragments as soluble proteins. Both N-terminal domains and a C-terminal peptide sequence from the alpha-subunit are essential for ligand binding with moderate affinity. However, binding of ligand with high affinity and specificity requires higher-order structure. To compare signalling capacities, we constructed chimaeras containing intracellular domains of insulin or IGF receptors fused to the extracellular portion of TrkC. Expression and activation of these chimaeras in cell lines reveals subtle differences in signalling and end-point responses, which may depend on cell background.

Insulin and insulin-like growth factors (IGFs) elicit distinct but overlapping biological effects in vivo. To investigate whether differences in intrinsic signaling capacity of receptors contribute to biological specificity, we constructed chimeric receptors containing the extracellular portion of the neurotrophin receptor TrkC fused to the intracellular portion of the insulin or IGF-I receptors. Chimeras were stably expressed in 3T3-L1 adipocytes at levels comparable to endogenous insulin receptors and were efficiently activated by neurotrophin-3. The wild-type insulin receptor chimera mediated approximately 2-fold greater phosphorylation of insulin receptor substrate 1 (IRS-1), association of IRS-1 with phosphoinositide 3-kinase, stimulation of glucose uptake, and GLUT4 translocation, compared with the IGF-I receptor chimera. In contrast, the IGF-I receptor chimera mediated more effective Shc phosphorylation, association of Shc with Grb2, and activation of mitogenactivated protein kinase compared with the insulin receptor chimera. The two receptors elicited similar activation of protein kinase B, p70S6 kinase, and glycogen synthesis. We conclude that the insulin receptor mediates some aspects of metabolic signaling in adipocytes more effectively than the IGF-I receptor, as a consequence of more efficient phosphorylation of IRS-1 and greater recruitment/activation of phosphoinositide 3-kinase. Insulin and insulin-like growth factors (IGFs)1 are structurally related polypeptides that exert diverse effects on mammalian tissues. The most prominent actions of insulin in vivo are concerned with the acute regulation of carbohydrate and lipid metabolism in liver, muscle, and fat, whereas IGFs act on skeletal and other tissues to promote growth, differentiation, and survival. The receptors for insulin and IGFs, which mediate these effects (IR and IGFR), are also structurally related and highly homologous, consisting of extracellular ␣-subunits responsible for ligand binding and transmembrane ␤-subunits possessing protein-tyrosine kinase activity, in a disulphide linked ␤-␣-␣-␤ configuration (1-3). Within the intracellular portion, the level of sequence identity between the receptors is greatest in the tyrosine kinase domain (84%) and somewhat less in the flanking juxtamembrane and carboxyl-terminal regions (61 and 44%, respectively). Not surprisingly, the signaling mechanisms of the insulin receptor (IR) and IGF-I receptor (IGFR) are broadly similar. Ligand binding activates tyrosine kinase activity, leading to phosphorylation of intracellular substrates, such as IRS and Shc proteins, and the recruitment and/or stimulation of signal transducing molecules, including phosphoinositide 3-kinase (PI 3-kinase) and Grb2⅐Sos (4, 5). These signal transducers in turn promote activation of proteinserine kinase cascades involving phosphoinositide-dependent kinase/PKB and MAPK/Erk kinase/MAPK, respectively, which modulate the activity of glucose transporters, enzymes, and transcription factors (6, 7).Given the similarity in structu...