XopM, a FFAT motif containing type-III effector protein from Xanthomonas, suppresses PTI responses at the plant plasma membrane

Many Gram-negative pathogenic bacteria use type-III effector proteins (T3Es) as essential virulence factors to suppress host immunity and to cause disease. However, in many cases the molecular function of T3Es remains unknown. The plant pathogen Xanthomonas campestris pv. vesicatoria (Xcv) is the causal agent of bacterial spot disease on tomato and pepper plants and is known to translocate around 30 T3Es into its host cell, which collectively suppress plant defense and promote infection. XopM is an Xcv core T3E with unknown function that has no similarity to any other known protein. We found that XopM interacts with vesicle-associated membrane protein (VAMP)-associated proteins (VAPs) in an isoform specific manner. The endoplasmic reticulum (ER) integral membrane protein VAP is a common component of membrane contact sites involved in both tethering and lipid transfer by binding directly to proteins containing a FFAT [two phenylalanines (FF) in an acidic tract (AT)] motif. Sequence analyses revealed that XopM displays two FFAT motifs that cooperatively mediated the interaction of XopM with VAP. When expressed in plants, XopM supports growth of a non-pathogenic bacterial strain and dampens the production of reactive oxygen species, indicating its ability to suppress plant immunity. Further analyses revealed that the interaction with VAP and the ability to suppress PTI are structurally and functionally separable. Although XopM requires localization to the host membrane system for full PTI suppression activity. We discuss a working model in which XopM uses FFAT-motifs to target the membrane in order to interfere with early PTI responses.