proline-alanine-alanine-arginine (PAAR) domain with C-terminal extension
This domain is found in the PAAR (proline-alanine-alanine-arginine) repeat family of mostly gamma-proteobacteria, and forms a sharp conical extension on the VgrG spike, a trimeric protein complex of the bacterial type VI secretion system (T6SS). Some members contains C-terminal domain extensions corresponding to Rearrangement hotspot (Rhs) protein repeats and conserved Rhs repeat-associated unique core sequences as well as uncharacterized domains. However, these terminal domains are exposed to solution, and do not distort the binding site of VgrG. Rhs and related YD-peptide repeat proteins are widely distributed in bacteria. Rhs shares similar architecture with distantly related WapA proteins of Bacillus and Listeria species, suggesting intercellular growth inhibition as its primary function. Additionally, a plasmid-encoded Rhs protein has been implicated in bacteriocin production in Pseudomonas savastanoi. 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. 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.
Feature 1:putative Zn binding site [ion binding site]
Evidence:
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
Comment:In this subfamily, most members contain only three out of four Zn binding residues.
Jiyao W et al.(2023). "The conserved domain database in 2023.",