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proline-alanine-alanine-arginine (PAAR) domain with uncharacterized C-terminal extension This domain is found in the PAAR (proline-alanine-alanine-arginine) repeat family of mostly beta- and gamma-proteobacteria, and forms a sharp conical extension on the VgrG spike, a trimeric protein complex of the bacterial type VI secretion system (T6SS). Most members contain C-terminal domain extensions corresponding to several uncharacterized domains such as S-type pyocin, DUF2235, DUF2345 and cytotoxic proteins. However, these terminal domains are exposed to solution, and do not distort the binding site of VgrG. 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. |
Jiyao W et al.(2023). "The conserved domain database in 2023.",