This family includes the inhibitor Spi and the pro-peptides of streptopain (SpeB). SpeB is produced as a 43 kDa pre-pro-protein, which is secreted via the recently described Sec secretory pathway Exportal. There is tight coupling between this inhibitor and its associated protease: the gene for the inhibitor Spi is located directly downstream from the gene for the streptococcal cysteine protease SpeB, and the sequence of the inhibitor is very similar to that of the SpeB propeptide. This is an example of an inhibitor molecule that is a structural homologue of the cognate propeptide, and is genetically linked to the protease gene [1].

1: SpeB-Spi: a novel protease-inhibitor pair from Streptococcus pyogenes. Kagawa TF, O'toole PW, Cooney JC; Mol Microbiol. 2005;57:650-666. PMID:16045611