Members of the CAAX-PSI family are proteases (after a isoprenyl group is attached to the Cys residue in the C-terminal CAAX motif of a protein to attach it to the membrane, the AAX tripeptide being removed by one of the CAAX prenyl proteases) (Pei and Grishin 2001). The family contains the CAAX prenyl protease. The proteins contain a highly conserved Glu-Glu motif at the amino end of the alignment. The alignment also contains two histidine residues that may be involved in zinc binding. While they are involved in membrane anchoring of proteins in eukaryotes, little is known about their function in prokaryotes (Pei and Grishin 2001). In some known bacteriocin loci, Abi genes have been found downstream of bacteriocin structural genes where they are probably involved in self-immunity (Kjos et al. 2010). Investigation of the bacteriocin-like loci in the Gram positive bacterial locus from Lactobacillus sakei 23K confirmed that the bacteriocin-like genes (sak23Kalphabeta) exhibited antimicrobial activity when expressed in a heterologous host and that the associated Abi gene (sak23Ki) conferred immunity against the cognate bacteriocin. Interestingly, the immunity genes from three similar systems conferred a high degree of cross-immunity against each other's bacteriocins, suggesting the recognition of a common receptor. Site-directed mutagenesis demonstrated that the conserved motifs constituting the putative proteolytic active site of the Abi proteins are essential for the immunity function of Sak23Ki - thus a new concept in self-immunity (Kjos et al. 2010).