A reversibly dissociable ternary complex formed by XpsL, XpsM and XpsN of the Xanthomonas campestris pv. campestris type II secretion apparatus

Abstract

The cytoplasmic membrane proteins XpsL, XpsM and XpsN are components required for type II secretion in Xanthomonas campestris pv. campestris. We performed metal-chelating chromatography to partially purify the His(6)-tagged XpsM (XpsMh)-containing complex. Immunoblot analysis revealed that both XpsL and XpsN co-eluted with XpsMh. The co-fractionated XpsL and XpsN proteins co-immune precipitated with each other, suggesting the existence of an XpsL-XpsM-XpsN complex. Ternary complex formation does not require other Xps protein components of the type II secretion apparatus. Further purification upon size-exclusion chromatography revealed that XpsN is prone to dissociate from the complex. Reassociation of XpsN with the XpsL-XpsMh complex immobilized on a nickel column is more effective than with XpsMh alone. Membrane-mixing experiments suggested that the XpsL-XpsMh complex and XpsN probably dissociate and reassociate in the membrane vesicles. Comparison of the half-lives of the XpsL-XpsMh-XpsN and XpsL-XpsMh complexes revealed that XpsL dissociates from the latter at a faster rate than from the former. Dissociation and reassociation between XpsL and XpsM were also demonstrated with membrane-mixing experiments. A dynamic model is proposed for the XpsL-XpsM-XpsN complex.