The AAA+ ATPases and HflB/FtsH Proteases of ‘Candidatus Phytoplasma mali’: Phylogenetic Diversity, Membrane Topology, and Relationship to Strain Virulence

Abstract

Previous examination revealed a correlation of phytopathogenic data of ‘Candidatus Phytoplasma mali’ strains and the DNA sequence variability of a type ATP00464 hflB gene fragment. To further investigate such a relationship, all distinct genes previously annotated as hflB in the genome of ‘Ca. P. mali’ strain AT were fully sequenced and analyzed from a number of representative mild, moderate, and severe strains. The re-annotation indicated that the sequences encode six AAA+ ATPases and six HflB proteases. Each of the nine distinct deduced AAA+ proteins that were examined formed a coherent phylogenetic cluster. However, within these groups, sequences of three ATPases and three proteases from mild and severe strains clustered distantly, according to their virulence. This grouping was supported by an association with virulence-related amino acid substitutions. Another finding was that full-length genes from ATPase AP11 could only be identified in mild and moderate strains. Prediction of the membrane topology indicated that the long ATPase- and protease-carrying C-terminal tails of approximately half of the AAA+ proteins are extracellular, putatively facing the environment of the sieve tubes. Thus, they may be involved in pathogen–host interactions and may compromise phloem function, a major effect of phytoplasma infection. All full-length genes examined appear transcriptionally active and all deduced peptides show the key positions indicative for protein function.