Abstract
Many bacterial pathogens employ the type III secretion system (T3SS) to translocate effector proteins into eukaryotic cells to overcome host defenses. To date, most of our knowledge about the T3SS molecular architecture comes from the studies on animal pathogens. In plant pathogens, nine Hrc proteins are believed to be structural components of the T3SS, of which HrcC and HrcJ form the outer and inner rings of the T3SS, respectively. Here, we demonstrated that a novel outer membrane-bound protein (HpaM) of Xanthomonas campestris pv. campestris is critical for the type III secretion and is structurally and functionally conserved in phytopathogenic Xanthomonas spp. We showed that the C-terminus of HpaM extends into the periplasm to interact physically with HrcJ and the middle part of HpaM interacts physically with HrcC. It is clear that the outer and inner rings compose the main basal body of the T3SS apparatus in animal pathogens. Therefore, we presume that HpaM may act as a T3SS structural component, or play a role in assisting assembling or affecting the stability of the T3SS apparatus. HpaM is a highly prevalent and specific protein in Xanthomonas spp., suggesting that the T3SS of Xanthomonas is distinctive in some aspects from other pathogens.
Highlights
Many Gram-negative bacterial pathogens of plants and animals employ the type III secretion system (T3SS) to deliver effector proteins into host cells, where they manipulate host cellular pathways to benefit the pathogens and allow the bacteria to successfully multiply
We identified a novel outer membrane-bound protein that is involved in the hypersensitive response (HR) and pathogenicity of X. campestris pv. campestris (Xcc), which was designated as HpaM
HpaM is not involved in the regulation of the expression of hrp genes that encode the components of the T3SS machinery, but interacts with HrcC and HrcJ, the homologues of the components that compose the outer and inner rings of the T3SS basal body of all bacterial pathogens that possess a T3SS
Summary
Many Gram-negative bacterial pathogens of plants and animals employ the type III secretion system (T3SS) to deliver effector proteins into host cells, where they manipulate host cellular pathways to benefit the pathogens and allow the bacteria to successfully multiply. It is presumed that the Hrc proteins are the components of the T3SS in plant pathogens and the core T3SS apparatus may be conserved among plant and animal pathogens[6,11] According to their homology to the T3SS components of animal pathogens, the function of the nine conserved Hrc proteins is believed to be: (1) HrcC is an outer membrane ring protein; (2) HrcJ is an inner membrane ring protein; (3) HrcR, S, T and U are integral inner membrane proteins with periplasmic extensions, taking part in the rod formation of the T3SS apparatus; and (4) HrcV, Q and N are inner membrane or peripheral cytoplasmic proteins engaged in initiation of effector secretion from the cytoplasm[6,8,11,12,13,14]. We present evidences showing that the protein is essential for type III secretion and conserved in Xanthomonas spp
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