Somatomedin B

Somatomedin B is a serum factor of unknown function, is a small cysteine-rich peptide, derived proteolytically from the N-terminus of the cell-substrate adhesion protein vitronectin.^[1] Cys-rich somatomedin B-like domains are found in a number of proteins,^[2] including plasma-cell membrane glycoprotein (which has nucleotide pyrophosphate and alkaline phosphodiesterase I activities)^[3] and placental protein 11 (which appears to possess amidolytic activity).

The SMB domain of vitronectin has been demonstrated to interact with both the urokinase receptor and the plasminogen activator inhibitor-1 (PAI-1) and the conserved cysteines of the NPP1 somatomedin B-like domain have been shown to mediate homodimerization.^[4] As shown in the following schematic representation below the SMB domain contains eight Cys residues, arranged into four disulfide bonds. It has been suggested^{[by whom?]} that the active SMB domain may be permitted considerable disulfide bond heterogeneity or variability, provided that the Cys25-Cys31 disulfide bond is preserved. The three-dimensional structure of the SMB domain is extremely compact and the disulfide bonds are packed in the centre of the domain forming a covalently bonded core.^[5] The structure of the SMB domain presents a new protein fold, with the only ordered secondary structure being a single-turn alpha-helix and a single-turn 3(10)-helix.^[6]

        xxCxxxxxxCxxxxxxxxxCxCxxxCxxxxxCCxxxxxCxxxxx                           ********************

'C': conserved cysteine probably involved in a disulfide bond.'*': position of the pattern.