This family of proteins represents the bacterial effector protein AvrPto from Pseudomonas syringae. This is the central core region of the protein which consists of a three-helix bundle motif. AvrPto is part of a type III secretion system from P.syringae which is involved in the bacterial speck disease of tomato. In resistant plants, AvrPto interacts with the host Pto kinase, which elicits an antibacterial defense response. In plants lacking resistance, the Pto kinase is not present and AvrPto acts as a virulence factor, promoting bacterial growth [1].

1: The solution structure of type III effector protein AvrPto reveals conformational and dynamic features important for plant pathogenesis. Wulf J, Pascuzzi PE, Fahmy A, Martin GB, Nicholson LK; Structure. 2004;12:1257-1268. PMID:15242602