This domain is found in the PAAR (proline-alanine-alanine-arginine) repeat superfamily, where it forms a sharp conical extension on the VgrG spike, a trimeric protein complex of the bacterial type VI secretion system (T6SS). The T6SS is responsible for translocation of a wide variety of toxic effector molecules, allowing predatory cells to kill prokaryotic as well as eukaryotic prey cells. The pointed tip of the PAAR domain is stabilized by a zinc atom positioned close to the cone's vertex and is likely to be important for its integrity during penetration of the target cell envelope. The PAAR-repeat proteins form a diverse superfamily with several subgroups extended both N- and C-terminally by domains with various predicted functions; the termini are exposed to solution, and do not distort the VgrG binding site. VgrG proteins are orthologous to the central baseplate spikes of bacteriophages with contractile tails, and genes encoding proteins with PAAR motifs have been frequently found immediately downstream from vgrG-like genes. It has been shown that PAAR proteins are essential for T6SS-mediated secretion and target cell killing by Vibrio cholerae (encodes two PAAR proteins) and Acinetobacter baylyi (encodes three PAAR proteins); inactivation of all these PAAR genes results in inactivation of Hcp secretion as well as T6SS-dependent killing of E. coli.
Comment:The Zn-binding site consists of three histidines and one cysteine in close homologs, and replaced with similar or complementary metal-binding residues such as arginines, lysines and glutamines in more distant homologs
Structure:4JIW: Escherichia coli PAAR-repeat protein binds Zn2+; contacts at 4.0A
Jiyao W et al.(2023). "The conserved domain database in 2023.",