Cloning, expression, purification and activation by Na ion of halophilic alkaline phosphatase from moderate halophile Halomonas sp. 593

We have succeeded in the cloning of alkaline phosphatase gene, haalp, from moderate halophile Halomonas sp. 593. A deduced amino acid sequence showed a high ratio of acidic to basic amino acids, characteristic of halophilic proteins. The gene product was efficiently expressed in Escherichia coli BL21 Star (DE3) pLysS, but in an inactive form. The purified recombinant HaALP was separated into four fractions by gel filtration. When they were dialyzed against 50 mM Tris–HCl (pH 8.0)/2 mM MgCl2 buffer containing 3 M NaCl, one of these four fractions was activated to almost full activity. This fraction contained a folding intermediate that was converted to the native structure by the salt. Among the additional salts tested, i.e., KCl, KBr, LiCl, MgCl2, (NH4)2SO4, and Na2SO4, only Na2SO4 was effective, suggesting the importance of Na ion.