Glutaraldehyde crosslinking analysis of the C12E8 solubilized H,K-ATPase

Transmembrane topology and oligomeric nature of an astrocytic membrane protein, MLC1

MLC1 is a membrane protein mainly expressed in astrocytes, and genetic mutations lead to the development of a leukodystrophy, megalencephalic leukoencephalopathy with subcortical cysts disease. Currently, the biochemical properties of the MLC1 protein are largely unknown. In this study, we aimed to characterize the transmembrane (TM) topology and oligomeric nature of the MLC1 protein. Systematic immunofluorescence staining data revealed that the MLC1 protein has eight TM domains and that both the N- and C-terminus face the cytoplasm. We found that MLC1 can be purified as an oligomer and could form a trimeric complex in both detergent micelles and reconstituted proteoliposomes. Additionally, a single-molecule photobleaching experiment showed that MLC1 protein complexes could consist of three MLC1 monomers in the reconstituted proteoliposomes. These results can provide a basis for both the high-resolution structural determination and functional characterization of the MLC1 protein.

Crystal structure of serine acetyl transferase from Brucella abortus and its complex with coenzyme A

Brucella abortus is the major cause of premature foetal abortion in cattle, can be transmitted from cattle to humans, and is considered a powerful biological weapon. De novo cysteine biosynthesis is one of the essential pathways reported in bacteria, protozoa, and plants. Serine acetyltransferase (SAT) initiates this reaction by catalyzing the formation of O-acetylserine (OAS) using l-serine and acetyl coenzyme A as substrates. Here we report kinetic and crystallographic studies of this enzyme from B. abortus. The kinetic studies indicate that cysteine competitively inhibits the binding of serine to B. abortus SAT (BaSAT) and noncompetitively inhibits the binding of acetyl coenzyme A. The crystal structures of BaSAT in its apo state and in complex with coenzyme A (CoA) were determined to 1.96Å and 1.87Å resolution, respectively. BaSAT was observed as a trimer in a size exclusion column; however, it was seen as a hexamer in dynamic light scattering (DLS) studies and in the crystal structure, indicating it may exist in both states. The complex structure shows coenzyme A bound to the C-terminal region, making mostly hydrophobic contacts from the center of the active site extending up to the surface of the protein. There is no conformational difference in the enzyme between the apo and the complexed states, indicating lock and key binding and the absence of an induced fit mechanism.

Active detergent-solubilized H+,K+-ATPase is a monomer

The H(+),K(+)-ATPase pumps protons or hydronium ions and is responsible for the acidification of the gastric fluid. It is made up of an α-catalytic and a β-glycosylated subunit. The relation between cation translocation and the organization of the protein in the membrane are not well understood. We describe here how pure and functionally active pig gastric H(+),K(+)-ATPase with an apparent Stokes radius of 6.3 nm can be obtained after solubilization with the non-ionic detergent C(12)E(8), followed by exchange of C(12)E(8) with Tween 20 on a Superose 6 column. Mass spectroscopy indicates that the β-subunit bears an excess mass of 9 kDa attributable to glycosylation. From chemical analysis, there are 0.25 g of phospholipids and around 0.024 g of cholesterol bound per g of protein. Analytical ultracentrifugation shows one main complex, sedimenting at s(20,)(w) = 7.2 ± 0.1 S, together with minor amounts of irreversibly aggregated material. From these data, a buoyant molecular mass is calculated, corresponding to an H(+),K(+)-ATPase α,β-protomer of 147.3 kDa. Complementary sedimentation velocity with deuterated water gives a picture of an α,β-protomer with 0.9-1.4 g/g of bound detergent and lipids and a reasonable frictional ratio of 1.5, corresponding to a Stokes radius of 7.1 nm. An α(2),β(2) dimer is rejected by the data. Light scattering coupled to gel filtration confirms the monomeric state of solubilized H(+),K(+)-ATPase. Thus, α,β H(+),K(+)-ATPase is active at least in detergent and may plausibly function as a monomer, as has been established for other P-type ATPases, Ca(2+)-ATPase and Na(+),K(+)-ATPase.

High-level expression, functional reconstitution, and quaternary structure of a prokaryotic ClC-type chloride channel

ClC-type anion-selective channels are widespread throughout eukaryotic organisms. BLAST homology searches reveal that many microbial genomes also contain members of the ClC family. An Escherichia coli-derived ClC Cl(-) channel homologue, "EriC," the product of the yadQ gene, was overexpressed in E. coli and purified in milligram quantities in a single-step procedure. Reconstitution of purified EriC into liposomes confers on these membranes permeability to anions with selectivity similar to that observed electrophysiologically in mammalian ClC channels. Cross-linking studies argue that EriC is a homodimer in both detergent micelles and reconstituted liposomes, a conclusion corroborated by gel filtration and analytical sedimentation experiments.

Glu-857 moderates K+-dependent stimulation and SCH 28080-dependent inhibition of the gastric H,K-ATPase

The rabbit H,K-ATPase alpha- and beta-subunits were transiently expressed in HEK293 T cells. The co-expression of the H,K-ATPase alpha- and beta-subunits was essential for the functional H,K-ATPase. The K+-stimulated H,K-ATPase activity of 0.82 +/- 0.2 micromol/mg/h saturated with a K0.5 (KCl) of 0.6 +/- 0.1 mM, whereas the 2-methyl-8-(phenylmethoxy)imidazo[1,2a]pyridine-3-acetonitrile (SCH 28080)-inhibited ATPase of 0.62 +/- 0.07 micromol/mg/h saturated with a Ki (SCH 28080) of 1.0 +/- 0.3 microM. Site mutations were introduced at the N,N-dicyclohexylcarbodiimide-reactive residue, Glu-857, to evaluate the role of this residue in ATPase function. Variations in the side chain size and charge of this residue did not inhibit the specific activity of the H,K-ATPase, but reversal of the side chain charge by substitution of Lys or Arg for Glu produced a reciprocal change in the sensitivity of the H,K-ATPase to K+ and SCH 28080. The K0.5 for K+stimulated ATPase was decreased to 0.2 +/-.05 and 0.2 +/-.03 mM, respectively, in Lys-857 and Arg-857 site mutants, whereas the Ki for SCH 28080-dependent inhibition was increased to 6.5 +/- 1.4 and 5.9 +/- 1.5 microM, respectively. The H,K-ATPase kinetics were unaffected by the introduction of Ala at this site, but Leu produced a modest reciprocal effect. These data indicate that Glu-857 is not an essential residue for cation-dependent activity but that the residue influences the kinetics of both K+ and SCH 28080-mediated functions. This finding suggests a possible role of this residue in the conformational equilibrium of the H,K-ATPase.

Transmembrane carboxyl residues are essential for cation-dependent function in the gastric H,K-ATPase

The K+-dependent ATPase activity of the H,K-ATPase was irreversibly inhibited by the carboxyl activating reagent, dicyclohexylcarbodiimide (DCCD). The inhibition was first order and displayed a concentration dependence with the K0.5 (DCCD) = 0.65 +/- 0.04 mM. KCl protected 70% of the ATPase activity from DCCD-dependent inhibition in a concentration-dependent manner with a K0.5 (K+) = 0.58 +/- 0.1 mM KCl. DCCD modification selectively inhibited the K+-dependent rather than ATP-dependent partial reactions including eosin fluorescence responses and ligand-stabilized initial tryptic cleavage patterns of the membrane-associated enzyme. DCCD modification also inhibited the binding of 86Rb+ and the fluorescent responses of the K+-competitive, fluorescent inhibitor 1-(2-methylphenyl)-4-methylamino-6-methyl-2, 3-dihydropyrrolo[3,2-c]quinoline. [14C]DCCD was incorporated into the H,K-ATPase in a time course identical to that describing the inactivation of the K+-dependent ATPase activity of the H,K-ATPase. A component of the [14C]DCCD incorporated into the H,K-ATPase was K+-sensitive where K+ reduced the [14C]DCCD incorporated into the enzyme by 1.6 nmol of [14C]DCCD/mg of protein. Membrane-associated tryptic peptides resolved from the [14C]DCCD-modified H,K-ATPase exhibited various K+ sensitivities with peptides at 23, 9.6, 8.2, 7.1, and 6.1 kDa containing 10-78%, 23-52%, 24-36%, 2%, and 3-4% K+-sensitivity, respectively. The N-terminal sequence of the K+-sensitive, approximately 23- and 9.6-kDa peptides was LVNE857, a C-terminal fragment of the ATPase alpha-subunit. The mass of the smaller peptide limited the residue assignment to the transmembrane M7/M8 domains and an intervening extracytoplasmic loop. An N-terminal sequence, SD840IM, was obtained from a 3.3-kDa, [14C]DCCD-labeled peptide resolved from a V8 digest of the partially purified alpha-subunit. This mass was sufficient to include LVNE but would exclude M8 and the intervening loop between M7 and M8. Glu857 is a unique residue present in each of the proteolytic preparations of the H,K-ATPase modified by [14C]DCCD. These data provide functional evidence of the selective inactivation of the K+-dependent partial reactions of the H,K-ATPase and show that Glu857 located at the M7 boundary in the C terminus of the pump molecule is a significant site of DCCD modification. These data are interpreted to indicate that this carboxyl residue is important for cation binding function.

Identification of H+/K(+)-ATPase alpha,beta-heterodimers

Glutaraldehyde treatment of the C12E8 solubilized H+/K(+)-ATPase crosslinks the catalytic subunit with an apparent molecular mass of 94 kDa in SDS polyacrylamide gels into two Coomassie stained particles migrating at approx. 147 and 173 kDa. The subunit composition of these particles was determined from the comparative distribution of FITC fluorescence, wheat germ agglutinin and anti-beta antibody reactivity in control and crosslinked preparations. FITC exclusively labelled the catalytic monomer of the native preparation and its fluorescence was initially distributed into two broad bands centered at approx. 147 and 173 kDa after crosslinking. These fluorescent bands coincided with the Coomassie stained particles. A glycoprotein(s) detected by wheat germ agglutinin reactivity was present in diffuse areas between 65 and 86 kDa and 95 to 134 kDa in the control preparation. This area was also labelled by the anti-beta antibodies. With crosslinking, the distribution of the wheat germ agglutinin reactive protein and anti-beta antibodies coincided with the crosslinked particles labelled by FITC. The presence of both the catalytic monomer and the beta subunit glycoprotein in the crosslinked particles indicated that these proteins were closely associated in the C12E8 solution. This suggests that the minimal structural particle of the H+/K(+)-ATPase is an alpha,beta-heterodimer.