Calnexin associates with Shaker K+ channel protein but is not involved in quality control of subunit folding or assembly.

Calnexin is part of an ER chaperone system that monitors and promotes the proper folding and assembly of glycosylated membrane proteins. To investigate the role of calnexin in the biogenesis of the voltage-dependent Shaker K+ channel, wild-type and mutant Shaker proteins were expressed in mammalian cells. Association with calnexin was assayed by coimmunoprecipitation.

Shaker and ether-à-go-go K+ channel subunits fail to coassemble in Xenopus oocytes.

Members of different voltage-gated K+ channel subfamilies usually do not form heteromultimers. However, coassembly between Shaker and ether-à-go-go (eag) subunits, members of two distinct K+ channel subfamilies, was suggested by genetic and functional studies (Zhong and Wu. 1991.

Membrane topology motifs in the SGLT cotransporter family.

Homologues of the Na+/glucose cotransporter, the SGLT family, include sequences of mammalian, eubacterial, yeast, insect and nematode origin. The cotransported substrates are sugars, inositol, proline, pantothenate, iodide, urea and undetermined solutes. It is reasonable to expect that the SGLT family members share a similar or identical topology of membrane spanning elements, by virtue of their common ancestry and similar coupling of solute transport to downhill sodium flux.

Specific dynamic action of a large carnivorous lizard, Varanus albigularis.

Varanus albigularis inhabits grasslands of southern and eastern Africa and experiences months of fasting during the dry season (May-December) followed by voracious feeding during the wet season (January-April). Previous studies have found that sit-and-wait foraging snakes, which also experience long intervals between large meals, exhibit unprecedented increases in post-feeding metabolism, which reflects the added cost of up-regulating a previously quiescent gut and digesting a large meal. Hence we measured pre- and post-prandial oxygen consumption rates (VO2) of adult V.

Regulation of Na+/glucose cotransporters.

Na+/glucose cotransporters (SGLTs) are expressed in the small intestine and the proximal renal tubule, where they play a central role in the absorption of glucose and galactose from food and the reabsorption of glucose from the glomerular filtrate. The regulation of intestinal sugar absorption occurs over two distinct time scales, one over days and the other over minutes. This review focuses on the mechanisms involved in the shorter-term regulation.

The human intestinal H+/oligopeptide cotransporter hPEPT1 transports differently-charged dipeptides with identical electrogenic properties.

The human intestinal H+/oligopeptide cotransporter hPEPT1, expressed in Xenopus oocytes, transported neutral, anionic and cationic dipeptides with identical electrogenic properties and maximal evoked currents. Currents were activated by 1 H+ regardless of the net charge on the driven substrate, and were independent of Na+o, K+i and Clo-, calling into question the familiar concept of the origin of the transporter-mediated current.

Intersubunit interaction between amino- and carboxyl-terminal cysteine residues in tetrameric shaker K+ channels.

Shaker potassium (K+) channels normally lack intrasubunit and intersubunit disulfide bonds. However, disulfide bonds are formed between Shaker subunits in intact cells exposed to oxidizing conditions. Upon electrophoresis under nonreducing conditions, intersubunit disulfide bond formation was detected by the presence of four high molecular weight adducts of Shaker protein.

Prenatal identification of a heterozygous status in two fetuses at risk for glucose-galactose malabsorption.

Glucose-galactose malabsorption (GGM) is an autosomal recessive disorder which presents with severe osmotic diarrhoea shortly after birth. Two proband siblings with GGM were previously demonstrated to contain a missense mutation (D28N) in the Na(+)-dependent glucose/galactose cotransporter (SGLT1) that accounts for the defect in sugar absorption. Prenatal screening for GGM was performed in two subsequent pregnancies in this large consanguineous family.

Defects in Na+/glucose cotransporter (SGLT1) trafficking and function cause glucose-galactose malabsorption.

Cotransporters harness ion gradients to drive 'active' transport of substrates into cells, for example, the Na+/glucose cotransporter (SGLT1) couples sugar transport to Na+ gradients across the intestinal brush border. Glucose-Galactose Malabsorption (GGM) is caused by a defect in SGLT1. The phenotype is neonatal onset of diarrhea that results in death unless these sugars are removed from the diet.

Kinetics and specificity of a H+/amino acid transporter from Arabidopsis thaliana.

The amino acid transporter AAP1/NAT2 recently cloned from Arabidopsis thaliana was expressed in Xenopus oocytes, and we used electrophysiological, radiotracer flux, and electron microscopic methods to characterize the biophysical properties, kinetics, and specificity of the transporter. Uptake of alanine was H(+)-dependent increasing from 14 pmol/oocyte/h at 0.032 microM H+ to 370 pmol/oocyte/h at 10 microM H+.

Membrane topology of the human Na+/glucose cotransporter SGLT1.

The membrane topology of the human Na+/glucose cotransporter SGLT1 has been probed using N-glycosylation scanning mutants and nested truncations. Functional analysis proved essential for establishment of signal-anchor topology. The resultant model diverges significantly from previously held suppositions of structure based primarily on hydropathy analysis.

Arginine-427 in the Na+/glucose cotransporter (SGLT1) is involved in trafficking to the plasma membrane.

To investigate the role of charged intramembrane residues in the function of the rabbit Na+/glucose cotransporter (rbSGLT1) we substituted arginine-427 (R427) by alanine in the putative domain M9 SGLT1. This residue is conserved in all the members of the SGLT1 family. The mutant protein (R427A) was expressed in Xenopus oocytes and, although Western blot analysis revealed that it was produced in amounts comparable to wild-type, no function was measured.

Kinetics of steady-state currents and charge movements associated with the rat Na+/glucose cotransporter.

The rat Na+/glucose cotransporter (SGLT1) was expressed in Xenopus oocytes and steady-state and transient currents were measured using a two-electrode voltage clamp. The maximal glucose induced Na(+)-dependent inward current was approximately 300-500 nA. The apparent affinity constants for sugar (alpha-methyl-D-glucopyranoside; alpha MDG) (K alpha MDG 0.