Geminivirus pathogenicity protein C4 interacts with Arabidopsis thaliana shaggy-related protein kinase AtSKη, a component of the brassinosteroid signalling pathway

Beet curly top virus (BCTV) C4 interacted with two members of the shaggy-related protein kinase family (AtSKη and AtSKζ) and a putative leucine-rich repeat receptor-like kinase (LRR-RLK) in a yeast two-hybrid assay. Tomato golden mosaic virus (TGMV) AC4 also bound with similar efficiency to AtSKη and AtSKζ but was unable to interact with the LRR-RLK. BCTV C4 interaction with AtSKη was confirmed using an in vitro binding assay. The protein kinases were capable of autophosphorylation in vitro and AtSKη phosphorylated BCTV C4 at threonine and serine residues. AtSKη phosphorylation of TGMV AC4 was significantly less efficient. The LRR-RLK did not efficiently phosphorylate BCTV C4. BCTV C4 localisation to the cell periphery in Nicotiana benthamiana was dependent on an intact N-terminal myristoylation motif, consistent with plasma membrane targeting. The intact motif was also required to produce the wild-type disease phenotype. Transient expression of BCTV C4 and TGMV AC4 derivatives in N. benthamiana identified additional amino acids within a central domain that contribute to the phenotype. The interaction with AtSKη indicates that BCTV C4 interacts with the brassinosteroid signalling pathway.