The structure of SMPI has been determined. It has 102 amino acid residues with two disulphide bridges and specifically inhibits metalloproteinases such as thermolysin , which belongs to MEROPS peptidase family M4. SMPI is composed of two beta-sheets , each consisting of four antiparallel beta-strands. The structure can be considered as two Greek key motifs with 2-fold internal symmetry, a Greek key beta-barrel . One unique structural feature found in SMPI is in its extension between the first and second strands of the second Greek key motif which is known to be involved in the inhibitory activity of SMPI. In the absence of sequence similarity, the SMPI structure shows clear similarity to both domains of the eye lens crystallins , both domains of the calcium sensor protein-S, as well as the single-domain yeast killer toxin . The yeast killer toxin structure was thought to be a precursor of the two-domain beta gamma-crystallin proteins, because of its structural similarity to each domain of the beta gamma-crystallins. SMPI thus provides another example of a single-domain protein structure that corresponds to the ancestral fold from which the two-domain proteins in the beta gamma-crystallin superfamily are believed to have evolved .