Nucleocytoplasmic partitioning of tobacco N receptor is modulated by SGT 1

SummarySGT 1 ( S uppressor of G2 allele of SKP 1) is required to maintain plant disease R esistance (R) proteins with N ucleotide‐ B inding ( NB ) and L eucine‐ R ich R epeat ( LRR ) domains in an inactive but signaling‐competent state. SGT 1 is an integral component of a multi‐protein network that includes RACK 1, Rac1, RAR 1, Rboh, HSP 90 and HSP 70, and in rice the M itogen‐ A ctivated P rotein K inase ( MAPK ), Os MAPK 6. Tobacco ( Nicotiana tabacum ) N protein, which belongs to the T oll‐ I nterleukin R eceptor ( TIR )‐ NB ‐ LRR class of R proteins, confers resistance to T obacco M osaic V irus ( TMV ). Following transient expression in planta , we analyzed the functional relationship between SGT 1, SIPK – a tobacco MAPK 6 ortholog – and N, using mass spectrometry, confocal microscopy and pathogen assays. Here, we show that tobacco SGT 1 undergoes specific phosphorylation in a canonical MAPK target‐motif by SIPK . Mutation of this motif to mimic SIPK phosphorylation leads to an increased proportion of cells displaying SGT 1 nuclear accumulation and impairs N‐mediated resistance to TMV , as does phospho‐null substitution at the same residue. Forced nuclear localization of SGT 1 causes N to be confined to nuclei. Our data suggest that one mode of regulating nucleocytoplasmic partitioning of R proteins is by maintaining appropriate levels of SGT 1 phosphorylation catalyzed by plant MAPK .