Stabilization of Na+,K+-ATPase purified from Pichia pastoris membranes by specific interactions with lipids

Na(+),K(+)-ATPase (porcine alpha(1)/His(10)*beta(1) or human alpha(1)/porcine His(10)*beta(1)) has been expressed in Pichia pastoris and purified by Co(2+)-chelate affinity resin chromatography, yielding about 80% pure, functional, and stable protein in a single step. The protein was eluted in nonionic detergents together with a. phosphatidylserine. Size exclusion chromatography showed that the protein eluted in n-dodecyl beta-D-maltoside is an alpha(1)/beta(1) protomer, whereas that in octaethylene glycol dodecyl monoether contains a mixture of alpha(1)/beta(1) protomer and higher order oligomers. The Na(+),K(+)-ATPase activity (8-16 (mu mol/min)/mg of protein) is similar in both detergents. Thus, the minimal functional unit is the alpha(1)/beta(1) protomer, and activity is unaffected by the presence of oligomeric forms. Screening of phospholipids for stabilization of the Na(+),K(+)-ATPase activity shows that (a) acid phospholipids are required and phosphatidylserine is somewhat better than phosphatidylinositol and (b) optimal stabilization is achieved with asymmetric phosphatidylserines having saturated (18:0 : 16:0) and unsaturated (18:1 > 18:2) side chains at sn-1 an sn-2 positions, respectively. In the presence of phosphatidylserine, cholesterol stabilizes the protein at 37 degrees C, but not at 0 degrees C. Cholesterol also increases the apparent affinity of the phosphatidylserine and stabilizes optimally in the presence of phosphatidylserines with a saturated fatty acyl chain at the sn-1 position. Ergosterol is a poor stabilizer. We propose that phosphatidylserine and cholesterol interact specifically with each other near the alpha(1)/beta(1) subunit interface, thus stabilizing the protein. These interactions do not seem to affect Na(+),K(+)-ATPase activity.