Purification of the membrane-spanning tryptic peptides of the .alpha. polypeptide from (sodium and potassium ion)-activated adenosine triphosphatase labeled with 1-tritiospiro[adamantane-4,1'-diazirine]

Abstract

Five long, membrane-spanning tryptic peptides from the .alpha. polypeptide of Na+ and K+ ATP [(Na+ + K+)-ATPase] were purified. (Na+ + K+)-ATPase, isolated from canine kidney, was exposed to UV light in the presence of a high concentration of 1-tritiospiro[adamantane-4,3''-diazirine], a carbene precursor that partitions into the bilayer of the membrane. The .alpha. polypeptide, modified with 1.2 mol of [3H]adamantylidene (mol of polypeptide)-1, was isolated and digested with trypsin. Digestion with trypsin ensures that membrane-spanning sequences remain intact during the digestion, since lysine and arginine being extremely hydrophilic rarely appear in the membrane-embedded regions of membrane proteins. This digestion produced radioactive tryptic peptides > 25 residues in length. The tryptic digest of the labeled .alpha. polypeptide was chromatographed on Sephadex LH-60 in ethanol-formic acid, 4:1. The majority of the radioactivity (87%) eluted with distribution coefficients corresponding to peptides longer than melittin (26 residues), whereas 73% of the protein traveled with distribution coefficients corresponding to peptides < 30 residues in length. Five radioactive peptides were further purified by high-pressure liquid chromatography, and each peptide displayed a unique, hydrophobic amino-terminal sequence. No other candidates could be found when a search for additional membrane-spanning peptides was conducted. Gel filtration of the tryptic peptides from the .alpha. polypeptide of (Na+ + K+)-ATPase labeled with 5-[125I]iodo-1-naphthyl azide, a lipophilic nitrene precursor, produced no additional radioactive components. Amino-terminal sequences and amino acid compositions of the 5 purified peptides are presented.