Expression of Na+,K+-ATPpase in Pichia pastoris analysis of wild type and D369N mutant proteins by Fe2+-catalyzed oxidative cleavage and molecular modeling

Na+,K+-ATPase (pig alpha1,beta1) has been expressed in the methylotrophic yeast Pichia pastoris. A protease-deficient strain was used, recombinant clones were screened for multicopy genomic integrants, and protein expression, and time and temperature of methanol induction were optimized. A 3-liter culture provides 300-500 mg of membrane protein with ouabain binding capacity of 30-50 pmol mg(-1). Turnover numbers of recombinant and renal Na+,K+-ATPase are similar, as are specific chymotryptic cleavages. Wild type (WT) and a D369N mutant have been analyzed by Fe2+- and ATP-Fe2+-catalyzed oxidative cleavage, described for renal Na+,K+-ATPase. Cleavage of the D369N mutant provides strong evidence for two Fe2+ sites: site 1 composed of residues in P and A cytoplasmic domains, and site 2 near trans-membrane segments M3/M1. The D369N mutation suppresses cleavages at site 1, which appears to be a normal Mg2+ site in E-2 conformations. The results suggest a possible role of the charge of Asp(369) on the E-1 <----> E-2 conformational equilibrium. 5'-Adenylyl-beta,gamma-imidodiphosphate(AMP-PNP)-Fe2+-catalyzed cleavage of the D369N mutant produces fragments in P ((VNDS)-V-712) and N (near (440)VAGDA) domains, described for WT, but only at high AMP-PNP-Fe2+ concentrations, and a new fragment in the P domain (near (367)CSDKTGT) resulting from cleavage. Thus, the mutation distorts the active site. A molecular dynamic simulation of ATP-Mg2+ binding to WT and D351N structures of Ca2+-ATPase (analogous to Asp(369) of Na+,K+-ATPase) supplies possible explanations for the new cleavage and for a high ATP affinity, which was observed previously for the mutant. The Asn(351) structure with bound ATP-Mg2+ may resemble the transition state of the WT poised for phosphorylation.