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【科普】研究揭示mRNA出核运输因子关键作用
Abstract Top
Nucleocytoplasmic trafficking is emerging as an important aspect of plant immunity. The three related pathways affecting plant immunity include Nuclear Localization Signal (NLS)–mediated nuclear protein import, Nuclear Export Signal (NES)–dependent nuclear protein export, and mRNA export relying on MOS3, a nucleoporin belonging to the Nup107–160 complex. Here we report the characterization, identification, and detailed analysis of Arabidopsis modifier of snc1, 11 (mos11). Mutations in MOS11 can partially suppress the dwarfism and enhanced disease resistance phenotypes of snc1, which carries a gain-of-function mutation in a TIR-NB-LRR type Resistance gene. MOS11 encodes a conserved eukaryotic protein with homology to the human RNA binding protein CIP29. Further functional analysis shows that MOS11 localizes to the nucleus and that the mos11 mutants accumulate more poly(A) mRNAs in the nucleus, likely resulting from reduced mRNA export activity. Epistasis analysis between mos3-1 and mos11-1 revealed that MOS11 probably functions in the same mRNA export pathway as MOS3, in a partially overlapping fashion, before the mRNA molecules pass through the nuclear pores. Taken together, MOS11 is identified as a new protein contributing to the transfer of mature mRNA from the nucleus to the cytosol.
Introduction Top
Plants utilize a two-layered immune system to recognize and combat pathogens. Conserved pathogen-associated molecular patterns (PAMPs), which are detected by plasma membrane localized plant pattern recognition receptors (PRRs), trigger a low amplitude defense response termed PAMP-triggered immunity (PTI) [1]. To suppress PTI, pathogens have evolved a suite of effectors (also called Avirulence or Avr proteins) [2]–[5]. The second layer of the plant immune response is initiated by the recognition of effectors or their effects, by cognate Resistance (R) proteins, leading to a strong defense response termed effector-triggered immunity (ETI) that culminates with hypersensitive response (HR) [1]: a programmed cell death event believed to restrict pathogen growth. Most cloned R genes encode Nucleotide Binding Leucine Rich-Repeat (NB-LRR) proteins with either a Toll/Interleukin1 receptor (TIR) or a Coiled-Coil (CC)-domain at their N terminus [6].
Mutant snc1 (suppressor of npr1-1, constitutive 1) plants carry a gain-of-function mutation in a TIR-NB-LRR R gene. This mutation renders the snc1 protein auto-active without the presence of pathogens. As a consequence, snc1 plants constitutively express Pathogenesis Related (PR) defense marker genes, accumulate high levels of defense phytohormone salicylic acid (SA), and are more resistant to the virulent bacterial pathogen Pseudomonas syringae maculicola (P.s.m.) ES4326 and the oomycete pathogen Hyaloperonospora arabidopsidis (H.a.) Noco2. Like most TIR-type R proteins, snc1 is fully dependent on EDS1 (Enhanced Disease Susceptibility 1) and PAD4 (PhytoAlexin Deficient 4) for its function [7], [8].
The snc1 mutant results from a point mutation in the linker region between the NB and LRR domains. This gain-of-function mutation causes activation of the ETI pathways mediated by TIR-NB-LRR R proteins. The extreme dwarf morphology of snc1 also makes it an ideal candidate to carry out genetic screens to investigate ETI components downstream of SNC1. Utilizing the unique autoimmune phenotypes of snc1, we conducted genetic screens to search for components contributing to R protein mediated immune responses. From the MOS (modifier of snc1) genetic screens, three nucleocytoplasmic pathways have been shown to affect plant immunity: Nuclear Localization Signal (NLS)-mediated nuclear protein import, Nuclear Export Signal (NES)-dependent nuclear protein export and mRNA export. MOS6, an importin α homolog involved in NLS-dependent protein import was shown to contribute to plant immunity [9]. More recently we identified MOS7, an integral nuclear pore component homologous to Nucleoporin88 (Nup88). Partial loss-of-function mos7-1 mutant plants showed increased NES-dependent protein export and exhibited reduced nuclear accumulation of the important defense regulators EDS1, NPR1 (Non-expressor of Pathogenesis Related 1) and snc1 [10]. The contribution of mRNA export to plant immunity was demonstrated in the mos3 mutant which exhibits defects in basal and R protein mediated immunity [10], [11]. MOS3/SAR3/AtNup96 is an integral nuclear pore component in the conserved Nup107–160 complex and was shown to be required for mRNA export [12].
Here we report the identification of modifier of snc1, 11 (mos11), a T-DNA insertional mutant. Mutation in MOS11 partially suppresses all the autoimmune phenotypes of snc1. MOS11 encodes a protein with unknown function. However, it is evolutionarily conserved and shares homology with a human protein CIP29. Our data suggest that MOS11 localizes to the nucleus and functions in the same mRNA export pathway as MOS3.
Results Top
mos11 partially suppresses the autoimmune phenotypes of snc1
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作者:admin@医学,生命科学 2011-01-29 12:40
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