Structural and biosynthetic studies on the two molecular forms of the (Na+ + K+)-activated adenosine triphosphatase large subunit inArtemia salina nauplii

Abstract

The large subunit of (Na⁺ + K⁺)-activated ATPase from brine shrimp, Artemia salina, migrates as two bands in sodium dodecyl sulfate-polyacrylamide gels. The slower migrating band, as observed in neutral or alkaline gel systems, is designated α₁ and the faster, α₂. Structural and biosynthetic studies have been performed to determine if these two bands represent independent molecular forms or precursor products. Peptide mapping of partial proteolytic digests of α₁ and α₂ showed no distinguishable difference between them, whereas this technique produced very distinct differences in the large subunit derived from three different species. The two large subunit bands also behaved identically when cross linked with cupric phenanthroline either in the presence or absence of digitonin, whereas other proteins in these preparations were unaffected. The peptide mapping and cross-linking experiments demonstrate that α₁ and α₂ have identical or nearly identical primary and probably higher order structure. Their different mobilities may be due to post-translational modification leading, for example, to different oligosaccharide composition. During development of the brine shrimp nauplius, α₁ increases in relative abundance while α₂ decreases. NaH¹⁴CO₃ incorporation and pulse-chase experiments indicate that α₁ and α₂, as well as the small subunit of the brine shrimp (Na⁺ + K⁺)-activated ATPase, are synthesized at the same time during development and that all changes in the rates of synthesis of these subunits occur at the same time. The apparent rates of degradation of the subunits are also similar. These results are inconsistent with a precursor-product relationship between α₁ and α₂.