CHARACTERIZATION OF STRUCTURAL SEQUENCES IN THE CHICKEN OSTEOCALCIN GENE - EXPRESSION OF OSTEOCALCIN BY MATURING OSTEOBLASTS AND BY HYPERTROPHIC CHONDROCYTES IN-VITRO

Osteocalcin is one of the major noncollagenous proteins specific to mi neralized connective tissues of vertebrates, A cDNA clone encoding the chicken osteocalcin gene was isolated, and the complete coding sequen ce for the 97-amino-acid pre-pro-osteocalcin was deduced. The 48-amino -acid pre-pro-peptide contains the expected hydrophobic leader sequenc e and the dibasic Lys-Arg sequence preceding the NH2-terminal His of t he mature 49-amino-acid chicken osteocalcin, which is believed to be n ecessary for pro-peptide cleavage. The pro-peptide sequence also conta ins the expected motif of polar and hydrophobic residues, including Ph e at -16, which targets vitamin K-dependent gamma-carboxylation of the three specific Glu residues at positions 17, 21, and 24 in the mature protein, Northern blots of total RNA were prepared from embryonic and adult chicken tissues (bone, brain, heart, intestine, kidney, muscle) and probed with chicken osteocalcin cDNA, The appearance of a single 0.5 kb mRNA species confirms that bone is the major site of osteocalci n expression in vivo. In primary osteoblasts isolated from 17-day embr yonic chicken calvaria, an osteocalcin mRNA of similar size is express ed concurrently with culture mineralization in vitro, Hypertrophic cho ndrocytes from 12-day ventral vertebrae and from the cephalic half of 17-day caudal sternae also express osteocalcin mRNA, hut nonhypertroph ic chondrocytes from the caudal half of 17-day sternae do not express osteocalcin mRNA.