PROSITE documentation PDOC00535

Osteonectin domain signatures

The following extracellular proteins contain, in their C-terminal section, a domain of about 240 amino acids which is highly conserved [1]. These proteins are:

• SPARC (also known as Osteonectin or BM-40), a glycoprotein that appears to regulate cell growth through interactions with the extracellular matrix and cytokines. It binds calcium and copper, several types of collagen, albumin, thrombospondin, PDGF and cell membranes. There are two calcium binding sites; a acidic domain that binds 5 to 8 Ca++ with a low affinity and a EF- hand loop that binds a Ca++ ion with a high affinity. SPARC is expressed at high levels in tissues undergoing morphogenesis, remodeling and repair.
• SC1, a mammalian brain extracellular matrix protein.
• QR1, an avian protein, specifically expressed in the neuroretina.

This domain has a N-terminal section of about 90 residues that contains 11 conserved cysteines, a central section that is probably in an α-helical conformation, and a C-terminal section that contains two EF-hand type calcium-binding regions [2] and three conserved cysteines. This topology is schematically represented below.

                                                                     +----+
                                                                     |    |
 xCxCxCxxCCxCxxxCxxCxCxxxxxCxCxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx<Ca>xCxxC<Ca>Cxx
  ***********                   ***

'C' : conserved cysteines involved in disulfide bonds.
'Ca': EF-hand region (see <PDOC00018>).
'*' : position of the patterns.

We developed two signature patterns for this domain. One is located in the C-terminal cysteine-rich section, the other consists in a highly conserved stretch of 11 residues in the central section.