Calcium binding domains and calcium-induced conformational transition of SPARC/BM-40/osteonectin, an extracellular glycoprotein expressed in mineralized and nonmineralized tissues.

Journal Article (Journal Article)

SPARC, BM-40, and osteonectin are identical or very closely related extracellular proteins of apparent Mr 43,000 (Mr 33,000 predicted from sequence). They were originally isolated from parietal endoderm cells, basement membrane producing tumors, and bone, respectively, but are rather widely distributed in various tissues. In view of the calcium binding activity reported for osteonectin, we analyzed the SPARC sequence and found two putative calcium binding domains. One is an N-terminal acidic region with clusters of glutamic acid residues. This region, although neither gamma-carboxylated nor homologous, resembles the gamma-carboxyglutamic acid (Gla) domain of vitamin K dependent proteins of the blood clotting system in charge density, size of negatively charged clusters, and linkage to the rest of the molecule by a cysteine-rich domain. The other region is an EF-hand calcium binding domain located near the C-terminus. A disulfide bond between the E and F helix is predicted from modeling the EF-hand structure with the known coordinates of intestinal calcium binding protein. The disulfide bridge apparently serves to stabilize the isolated calcium loop in the extracellular protein. As observed for cytoplasmic EF-hand-containing proteins and for Gla domain containing proteins, a major conformational transition is induced in BM-40 upon binding of several Ca2+ ions. This is accompanied by a 35% increase in alpha-helicity. A pronounced sigmoidicity of the dependence of the circular dichroism signal at 220 nm on calcium concentration indicates that the process is cooperative. In view of its properties, abundance, and wide distribution, it is proposed that SPARC/BM-40/osteonectin has a rather general regulatory function in calcium-dependent processes of the extracellular matrix.

Full Text

Duke Authors

Cited Authors

  • Engel, J; Taylor, W; Paulsson, M; Sage, H; Hogan, B

Published Date

  • November 3, 1987

Published In

Volume / Issue

  • 26 / 22

Start / End Page

  • 6958 - 6965

PubMed ID

  • 3427055

International Standard Serial Number (ISSN)

  • 0006-2960

Digital Object Identifier (DOI)

  • 10.1021/bi00396a015


  • eng

Conference Location

  • United States