Abstract
Lipopolysaccharide (LPS) is a major constituent of the outer membrane of Gram-negative bacteria and acts as a pathogen-associated molecular pattern that triggers immune responses in both plants and animals. LPS-binding protein (LBP) and bactericidal/permeability-increasing protein (BPI), which bind to LPS and play important roles in immunity of mammals, have been well studied. However, the molecule contributing to LPS binding in plants is mostly unknown. The Arabidopsis genome carries two genes encoding LBP/BPI-related proteins which we designated as AtLBP/BPI related-1 (AtLBR-1) and AtLBP/BPI related-2 (AtLBR-2). We found that their N-terminal domains were co-purified with cell wall-derived LPS when expressed in E. coli. Since this finding implied the direct binding of AtLBRs to LPS, we also confirmed binding by using LPS-free AtLBRs and purified LPS. AtLBRs directly bind to both rough and smooth types of LPS. We also demonstrated that LPS-treated atlbr mutant Arabidopsis exhibit a significant delay of induction of defence-related gene pathogenesis-related 1 (PR1) but no other PR genes. Furthermore, LPS-treated atlbr mutants showed defects in reactive oxygen species (ROS) generation. These results demonstrate that, as well as LBP and BPI of mammals, AtLBRs also play an important role in the LPS-induced immune response of plants.
Highlights
Plants detect pathogen invasions by recognition of pathogen-associated molecular patterns (PAMPs)
Because it has been reported that the recombinant N-terminal domain of Human LBP (hLBP) was co-purified with LPS derived from expression host E. coli[19], we investigated if AtLBRs could be co-purified with LPS
We demonstrated that AtLBRs appear to have an immunological role similar to LPS-binding protein (LBP)/bactericidal/permeability-increasing protein (BPI) subfamily proteins, including mammalian LBP, BPI, and Cg-BPIs, and PLUNC-related protein OCX-36
Summary
Plants detect pathogen invasions by recognition of pathogen-associated molecular patterns (PAMPs). The binding of BPI to LPS increases the permeability of the bacterial membranes and opsonizes bacteria to enhance phagocytosis by neutrophils[5] Another important aspect of BPI is the attenuation of the LPS-induced inflammatory response by competitive inhibition against LBP5. The fact that members of both subfamilies (i.e., hLBP, hBPI, OCX-36 and Cg-BPI) bind to LPS and participate in innate immune responses against potential bacterial invasion motivated the current study to characterize plant proteins belonging to the LBP/BPI/ PLUNC superfamily. LPS-treated atlbr mutants showed the defect in immune responses, such as PR1 gene expression and ROS production These results demonstrate the biological importance of LBRs for induction of LPS-triggered defence responses in plants and the functional similarities among LBP/BPI subfamily from various organisms
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