The RXLR cytoplasmic effector AVR3a of Phytophthora infestans confers avirulence on potato plants carrying the R3a gene. Two alleles of Avr3a encode secreted
proteins that differ in only three
amino acid residues, two of which are in the mature
protein. Avirulent isolates carry the Avr3a allele, which encodes AVR3aKI (containing
amino acids C19, K80 and I103), whereas virulent isolates express only the virulence allele avr3a, encoding AVR3aEM (S19, E80 and M103). Only the AVR3aKI
protein is recognized inside the plant cytoplasm where it triggers R3a-mediated
hypersensitivity. Similar to other oomycete avirulence
proteins, AVR3aKI carries a
signal peptide followed by a conserved motif centered on the consensus RXLR sequence that is functionally similar to a host cell-targeting signal of
malaria parasites. The interaction between Avr3a and R3a can be reconstructed by their transient co-expression in Nicotiana benthamiana. We exploited the N. benthamiana experimental system to further characterize the Avr3a-R3a interaction. R3a activation by AVR3aKI is dependent on the
ubiquitin ligase-associated
protein SGT1 and
heat-shock protein HSP90. The AVR3aKI and AVR3aEM
proteins are equally stable in planta, suggesting that the difference in R3a-mediated death cannot be attributed to AVR3aEM
protein instability. AVR3aKI is able to suppress cell death induced by the elicitin INF1 of P. infestans, suggesting a possible virulence function for this
protein. Structure-function experiments indicated that the 75-amino
acid C-terminal half of AVR3aKI, which excludes the RXLR region, is sufficient for avirulence and suppression functions, consistent with the view that the N-terminal region of AVR3aKI and other RXLR effectors is involved in secretion and targeting but is not required for effector activity. We also found that both polymorphic
amino acids, K80 and I103, of mature AVR3a contribute to the effector functions.