A MAJOR NONCOLLAGENOUS 62-KDA PROTEIN FROM RAT BONE MINERALIZED MATRIX IS IDENTICAL TO PP(63), A PHOSPHORYLATED GLYCOPROTEIN FROM LIVER

A protein present as a M(r) 62 k monomer and as several differently sized disulfide-bonded oligomers has been isolated from rat bone mineralized matrix. Its overall tissue distribution determined by ELISA immunoassays showed the protein present only in bone, tooth and in serum while aorta, cartilage, intestine, kidney, liver, muscle, skin, spleen and tendon were all negative. Despite that the 62 kDa protein was abundant and selectively found in bone, no positive cDNA clone could be identified in several rat bone libraries. Positive clones were, however, identified in a rat liver expression library. A cDNA clone of 1.3 kb hybridized in a Northern blotting assay to a 1.8 kb mRNA in rat liver. No hybridization signal was detected with RNA from bone, brain, lung, muscle, spleen and kidney. Sequence analysis of the isolated cDNA clone revealed a 50-bp untranslated region followed by an open reading frame of 357 amino acids. The open reading frame can be divided into a 17-amino acid signal peptide followed by the mature protein of 340 amino acids with alanine as its N-terminal amino acid. A short N-terminal amino acid sequence from the isolated 62-kDa bone protein verified the molecular identity of the cDNA clone. The primary structure of the 62-kDa liver protein was identical to a that of a 63-kDa phosphorylated glycoprotein (pp63) from liver.