Labelling of the hydrophobic domain of the Na+,K+-ATPase

1-Palmitoyl-2-(p-benzoyl)benzoyl phosphatidylcholine, a photoactive phospholipid for the labelling of membrane components

The preparation and use of a new photoactivatable phosphatidylcholine derivative [1-palmitoyl-2-(p-benzoyl)benzoyl phosphatidylcholine] is described. The reagent is shown to be effective in labelling different membrane proteins under irradiation conditions that preserve their enzymic activity. The properties of this new phospholipid analogue suggest that it can be useful in studying several aspects of membrane structure and function.

Reconstitution and photolabeling of the purified (Na+ + Mg2+)-ATPase from the plasma membrane of Acholeplasma laidlawii B with phospholipids containing a photosensitive fatty acyl group

The purified membrane (Na+ + Mg2+)-ATPase of Acholeplasma laidlawii B was reconstituted into vesicles composed of phospholipids containing a photoactivatable aryl nitrene-generating fatty acyl group. The reconstitution with phospholipid resulted in an enhancement of ATPase activity and a reduction in the sensitivity of the enzyme to radiation inactivation. The incorporation of the enzyme into the lipid vesicles results in a broadening of the gel-to-liquid-crystalline phase transition of the photolabeled phospholipid and the appearance of two partially resolved endotherms in the calorimetric traces. The temperatures and the total enthalpy of these overlapping transitions are higher than in the absence of incorporated enzyme. After photolysis of the lipid-reconstituted ATPase and separation of the polypeptide subunits by sodium dodecyl sulfate (SDS) gel electrophoresis, a significant labeling of the alpha-subunit of the enzyme was demonstrated. These results indicate that at least the alpha-subunit of this ATPase must penetrate into or traverse the phospholipid bilayer.

Papain fragmentation of the (Na+,K+)-ATPase beta subunit reveals multiple membrane-bound domains

Purified dog kidney (Na+,K+)-ATPase was reacted with tritiated sodium borohydride after treatment with neuraminidase and galactose oxidase. This procedure did not affect the ATPase activity of the enzyme, and all of the covalently bound radioactivity was found in the beta subunit (Mr 54 000). Papain digestion of the tritiated enzyme produced two labeled fragments of Mr 40 000 and 16 000. Further proteolysis generated an Mr 31 000 peptide from the larger fragment. Unlike the tryptic and chymotryptic sites of the alpha subunit, the sites of papain hydrolysis were insensitive to conformations of the (Na+,K+)-ATPase. Determination of the NH2-terminal sequences was used to arrange the fragments within the linear map of the beta chain. Finally, none of the labeled peptides was released from the membrane under nondenaturing conditions. These results are consistent with a model of the beta subunit containing a 40 000-dalton NH2-terminal piece and a 16 000-dalton COOH-terminal piece. Both fragments have extracellularly exposed carbohydrate and at least one membrane-bound domain.

Photolabelling of D-beta-hydroxybutyrate apodehydrogenase with azidoaryl phospholipids

Two synthetic photoactive azidoarylphosphatidylcholines were used to investigate the level of interaction between D-beta-hydroxybutyrate apodehydrogenase (apoBDH), an amphipathic membrane protein, with the hydrophobic domain of phospholipids. The two synthetic lecithins, PL I (1-myristoyl-2-12-N-(4-azido-2-nitrophenyl) aminododecanoyl-sn-glycero-3-phosphocholine) and PL II (1-myristoyl-2-(2-azido-4-nitrobenzoyl)-sn-glycero-3-phosphocholine), are able to reactivate the non-active purified apoBDH as well as the non-photoactive homologs, indicating that the photoreactive chemical groups are without effect on the cofactor properties of phosphatidylcholine. Photoirradiation of reconstituted complexes between phospholipid containing azidoaryllecithin and apoBDH leads to a covalent binding of some synthetic lecithin molecules on the protein. The labelling, about 3 times higher with PL II than with PL I, suggests that the area of interacting domain of BDH with the hydrophobic moiety of phospholipid is more important at or near the surface of the lipid bilayer than in the inner part. This approach is further demonstration that BDH is an integral protein.

Micellar gangliosides mediate the lipid insertion of cholera toxin protomer A

The topology of the interaction of cholera toxin with ganglioside and detergent micelles was studied with the technique of hydrophobic photolabelling. Cholera toxin alpha and gamma polypeptide chains appear to penetrate into the hydrophobic core of ganglioside micelles. Micelles of SDS cause the labelling also of the beta polypeptide chains, while Triton X-100 micelles have little ability to mediate the labelling of the toxin. The specific reduction of the alpha-gamma disulfide bond allows the penetration of the alpha polypeptide chain into Triton X-100 micelles, but does not affect the interaction of cholera toxin with either ganglioside or SDS micelles. Thus, ganglioside micelles appear to cause a conformational change of the native toxin, such as to induce the penetration of the alpha chain into the micelle hydrophobic core.

A simple method for the purification of rat brain Na+,K+-adenosine triphosphatase (ATPase)

Several methods of purification of Na+,K+-adenosine triphosphatase (ATPase) have been previously described for a wide variety of tissues. In general, highest activity preparations have necessitated large amounts of tissue and many purification steps. This article describes a technique that allows partial purification of Na+,K+-ATPase from as few as 15 rat brains and should be of interest to investigators of the pharmacology of this particular enzyme system. In this modified version of the Jorgensen procedure (Biochim Biophys Acta 356:36--52, 1974) we purified the Na+,K+-ATPase from 15--90 rat brains, and obtained enzyme preparations with a mean specific activity of 552 +/- 37.6 mumol Pi/mg of protein/hr (95.5% ouabain sensitive). This "purified" enzyme had an activity ratio (Mg2+ + Na+ + K+)/(Mg2+ + Na+) of 47.4 +/- 12.3 SEM, compared to 3.29 +/- 0.17 SEM for the untreated microsomes. Ouabain inhibited the "purified" enzyme with an I50 of 6 X 10(-9) M. Ouabain binding (644 pmol/mg of protein) yielded a turnover number of 13,700 min-1. Sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis of the enzyme revealed predominantly the alpha and beta subunits with some minor contaminant bands. Previous methods of purification of rat brain Na+,K+-ATPase have employed sodium deoxycholate and high concentrations of NaI; the reported specific activity obtained was generally 150--350 mumol Pi/mg of protein/hr. We have employed higher SDS concentrations than in Jorgensen's technique for rabbit kidney but the procedure is simpler because sucrose gradients are not used. Final wash steps also include 10--20% glycerol in the media. These modifications have yielded Na+,K+-ATPase of significantly higher specific activity than previously reported for rat brain.

Immunochemical evidence for a transmembrane orientation of both the (Na+, K+)-ATPase subunits

Antibodies were raised against the large catalytic subunit (apparent Mr 96000) and the glycoprotein (apparent Mr 60000) of the sodium- and potassium-dependent adenosine triphosphatase [(Na+, K+)-ATPase] from Bufo marinus. The specificity of each antiserum was assessed by two-dimensional immunoelectrophoresis using toad kidney microsomes or the purified holoenzyme as a source of antigen and by indirect immunoprecipitation of detergent-solubilized (Na+, K+)-ATPase subunits from radioiodinated or biosynthetically labeled kidney holoenzyme, microsomes, or postnuclear supernatant. The anticatalytic subunit serum reacted exclusively with a 96000-dalton protein. The antiserum to the glycoprotein was rendered specific to this subunit by absorption with purified catalytic subunit. The two antisera were agglutinating and lytic in the presence of complement when toad erythrocytes were used as targets, indicating that antigenic determinants of both subunits were exposed on the cell surface. The specific reactivities with surface-exposed antigenic determinants of both subunits could be absorbed with toad red blood cells. Such absorbed antisera still reacted with detergent-treated or untreated kidney microsomes, revealing the presence of cytoplasmic and/or intramembranous antigenic sites. Our immunochemical data demonstrate that the glycoprotein subunit of (Na+, K+)-ATPase spans the lipid bilayer and confirm the transmembrane orientation of the catalytic subunit postulated from functional studies.