The cystatin superfamily consists of a large group of cystatin domain-containing proteins, most of which are reversible and tight-binding inhibitors of the papain (C1) and legumain (C13) families of cysteine proteases. Fetuins have been identified as main members of the cystatin superfamily and are composed of fetuin-A and fetuin-B. Fetuins are characterized by the presence of 2 N-terminally located cystatin-like repeats (see <PDOC00259>) and a unique C-terminal domain which is not present in other proteins of the cystatin family [1,2,3].

Fetuin-A [2,3,4,5,6] also called α-2-HS-glycoprotein, bone sialic acid-containing protein (BSP), countertrypin or PP63, is expressed in a tissue- and development-specific pattern which seems to be significantly different between species. A wide functional diversity of fetuin-A has been observed. It has been shown to function in many physiological aspects, such as fatty acid transport, regulation of insulin activity and hepatocyte-growth-factor activity, response to systemic inflammation, and inhibition of unwanted mineralization. It has been demonstrated that fetuin-A plays important roles during developmental processes, including osteogenesis, myotubule, fetal brain and nervous system development. Human fetuin is a heterodimer of chain A and B, which are derived by cleavage of a connecting peptide from a common precursor. Snake fetuin family proteins (antihemorrhagic proteins HSF, BJ46a and MSF and HLP-A and HLP-B) show a significant degree of sequence homology to fetuin-A [7].

Fetuin-B has been suggested to be involved in systemic inflammation, as a tumor suppressor, and as a basic calcium phosphate precipitation inhibitor [8,9].

The cystatin fold is formed by a five stranded anti-parallel β-sheet wrapped around a five-turn α-helix [1].

As shown in the schematic representation fetuin contains twelve conserved cysteines involved in six disulfide bonds.

   Chain A                                                   Chain B

              +--+  +---+  +-+         +--+  +---+
    **********|**|**|***|**|*|         |  |  |   |
   xCxxxxxxxxxCxxCxxCxxxCxxCxCxxxxxxxxxCxxCxxCxxxCxxxxxxxxx  xxxxxxxxCxx
    |        ***                                                     |
    +----------------------------------------------------------------+

'C': conserved cysteine involved in a disulfide bond.
'*': position of pattern 1 (upper line) and 2 (lower line).

Eleven of the twelve invariant cysteines are located within the cystatin-like repeats. This domain has been choosen to assign a signature pattern. The 12th cysteine is located near the C-terminus of the protein, separated by a region of variable length. A second signature pattern has been developed from a well conserved region around the 2nd invariant cysteine. We also developped two profiles, which cover respectively the fetuin-A- and fetuin-B-type cystatin domains.