Proteolytic processing of both RXLR and EER motifs in oomycete effectors

Abstract

Arg-any amino acid-Leu-Arg (RXLR) effectors are central oomycete virulence factors that target diverse host proteins and processes to suppress plant immunity. Relatively little is known about how they are processed post-translationally before delivery into host cells. Proteolytic cleavage at the RXLR motif was observed to occur prior to secretion in all Phytophthora infestans effectors tested, suggesting it is a general rule, and was observed to occur between the leucine and the second arginine. There was no cleavage of a naturally occurring second RXLR motif in a structured region of Pi21388/AvrBlb1, or one introduced at a similar position in effector Pi04314, in keeping with the motif being positionally constrained, potentially to disordered regions closely following the signal peptide. Remarkably, independent proteolytic cleavage of the Glu-Glu-Arg (EER) motif, often found immediately downstream of the RXLR, was also observed in diverse effectors, occurring immediately after the arginine. Expression of full-length effectors in host plant Nicotiana benthamiana revealed that, although secreted, they were poorly processed, suggesting that RXLR and EER cleavage does not occur in all eukaryotic cells. Our observations indicate that, whether possessing both RXLR and EER, or either motif alone, these effectors are likely proteolytically processed prior to secretion in all cases.