Studies of structure-function relationships in Na,KATPase require high yield expression of inactive mutations in cells without endogenous Na,K-ATPase activity. In this work we developed a host/vector system for expression of fully active pig Na,K-ATPase as well as the inactive mutations D369N and D807N at high levels in Saccharomyces cerevisiae. The ␣1-and ␤1-subunit cDNAs were inserted into a single 2-m-based plasmid with a high and regulatable copy number and strong galactose-inducible promoters allowing for stoichiometric alterations of gene dosage. The protease-deficient host strain was engineered to express high levels of GAL4 transactivating protein, thereby causing a 10-fold increase in expression to 32,500 ؎ 3,000

Obesity is a major threat to global health and metabolically associated with glycerol homeostasis. Here we demonstrate that in human adipocytes, the decreased pH observed during lipolysis (fat burning) correlates with increased glycerol release and stimulation of aquaglyceroporin AQP10. The crystal structure of human AQP10 determined at 2.3 Å resolution unveils the molecular basis for pH modulation—an exceptionally wide selectivity (ar/R) filter and a unique cytoplasmic gate. Structural and functional (in vitro and in vivo) analyses disclose a glycerol-specific pH-dependence and pinpoint pore-lining His80 as the pH-sensor. Molecular dynamics simulations indicate how gate opening is achieved. These findings unravel a unique type of aquaporin regulation important for controlling body fat mass. Thus, targeting the cytoplasmic gate to induce constitutive glycerol secretion may offer an attractive option for treating obesity and related complications.

The focus of this article is on progress in establishing structure-function relationships through site-directed mutagenesis and direct binding assay of Tl(+), Rb(+), K(+), Na(+), Mg(2+) or free ATP at equilibrium in Na,K-ATPase. Direct binding may identify residues coordinating cations in the E(2)[2K] or E(1)P[3Na] forms of the ping-pong reaction sequence and allow estimates of their contributions to the change of Gibbs free energy of binding. This is required to understand the molecular basis for the pronounced Na/K selectivity at the cytoplasmic and extracellular surfaces. Intramembrane Glu(327) in transmembrane segment M4, Glu(779) in M5, Asp(804) and Asp(808) in M6 are essential for tight binding of K(+) and Na(+). Asn(324) and Glu(327) in M4, Thr(774), Asn(776), and Glu(779) in 771-YTLTSNIPEITP of M5 contribute to Na(+)/K(+) selectivity. Free ATP binding identifies Arg(544) as essential for high affinity binding of ATP or ADP. In the 708-TGDGVND segment, mutations of Asp(710) or Asn(713) do not interfere with free ATP binding. Asp(710) is essential and Asn(713) is important for coordination of Mg(2+) in the E(1)P[3Na] complex, but they do not contribute to Mg(2+) binding in the E(2)P-ouabain complex. Transition to the E(2)P form involves a shift of Mg(2+) coordination away from Asp(710) and Asn(713) and the two residues become more important for hydrolysis of the acyl phosphate bond at Asp(369).

The sequence Y771TLTSNIPEIT781P in the fifth transmembrane segment of the alpha-subunit of Na,K-ATPase is unique among cation pump proteins. Here, in search of the molecular basis for Na,K specificity, alanine and conservative substitutions were directed to six oxygen-carrying residues in this segment. The contribution of the residues to cation binding was estimated from direct binding of Tl+ [Nielsen, et al. (1998) Biochemistry 37, 1961-1968], K+ displacement of ATP binding at equilibrium, and Na+-dependent phosphorylation from ATP in the presence of oligomycin. As an intrinsic control, substitution of Thr781 had no effect on Tl+(K+) or Na+ binding. There are several novel observations from this work. First, the carboxamide group of Asn776 is equally important for binding Tl+(K+) or Na+, whereas a shift of the position of the carboxamide of Asn776 (Asn776Gln) causes a large depression of Na+ binding without affecting the binding of Tl+(K+). Second, Thr774 is important for Na+ selectivity because removal of the hydroxyl group reduces the binding of Na+ with no effect on binding of Tl+(K+). Removal of the methyl groups of Thr774 or Thr772 reduces binding of both Tl+(K+) and Na+, whereas the hydroxyl group of Thr772 does not contribute to cation binding. Furthermore, the hydroxyl groups of Ser775 and Tyr771 are important for binding both Tl+(K+) and Na+. The data suggest that rotating or tilting of the cytoplasmic part of the fifth transmembrane segment may adapt distances between coordinating groups and contribute to the distinctive Na+/K+ selectivity of the pump.

The segment 708 TGDGVNDSPALKK 720 in the ␣-subunit P domain of Na,K-ATPase is highly conserved among cation pumps, but little is known about its role in binding of Mg 2؉ 710 3 Ala mutation also interferes with transmission of structural changes to the ouabain site and reduces the affinity for binding of Tl ؉ 2-to 3-fold, suggesting a role in transmission of K ؉ stimulation of phospho-enzyme hydrolysis from transmembrane segment 5 to the P domain.