Molecular analysis of chicken embryo SPARC (osteonectin)

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Abstract
SPARC is a secreted glycoprotein that modulates cell shape and cell‐matrix interactions. Levels of SPARC are increased at sites of somitogenesis, osteogenesis, and angiogenesis in the embryo and during wound repair in the adult. We have cloned and characterized SPARC from chicken embryo. A 2.2‐kbp cDNA, obtained by a novel use of the polymerase chain reaction, was determined to encode a 298‐residue protein that is 85% identical to human SPARC. Antigenic sites in particular appear to be highly conserved, as antibodies against C‐terminal sequences of murine and bovine SPARC reacted with a 41–43 kDa protein in chicken embryo extracts. Chicken SPARC can be defined by four sequence signatures: (a) a conserved spacing of 11 cysteine residues in domain II, (b) the pentapeptide KKGHK in domain II, which is contained within a larger region of 31 identical residues, (c) a 100% conserved region of 10 residues in domain III, and (d) a C‐terminal, calcium‐binding EF‐hand motif. SPARC mRNAs in the 10‐day‐old chicken embryo are represented by three sizes of 1.8, 2.2 and 3.0 kb. The relative steady‐state levels for the 2.2‐kb mRNA were determined as aorta ≥ skeletal muscle > calvarium > vertebra > anterior limb > kidney > heart > brain > skin and lung ≥ liver. The relative abundance of the 1.8‐kb and 2.2‐kb mRNAs varied among tissues and indicated that differential processing of SPARC mRNAs might occur. All three RNA species were detected by a cDNA probe for the N‐terminal part of the coding region. Thus, the three mRNA species appear to arise from differential 3′ splicing and/or polyadenylation. Collective evidence demonstrates that SPARC has been well‐conserved during vertebrate evolution, a finding that indicates a fundamental role for this protein in development.