Crystal structure of lpg1832, a VirK family protein from Legionella pneumophila, reveals a novel fold for bacterial VirK proteins.

VirK family [Pfam06903] consists of 14 bacterial VirK proteins of around 145 residues in length. The function of this family is unknown. Herein, using single-wavelength anomalous diffraction, we determined the crystal structure of lpg1832, a VirK family protein from Legionella pneumophila, at 2.0 Å resolution. This is the first structural determination of a VirK domain-containing protein. Lpg1832 is a type II secretion system-dependent extracellular protein that folds into a novel barrel-shaped structure. It is found to adopt a quaternary assembly comprising a homotetramer. The three-dimensional structure of lpg1832 provides the first structural information pertaining to the VirK family and allows us to possibly identify its functionally important regions.