title: The endosymbiont Amoebophilus asiaticus encodes an S-adenosylmethionine carrier that compensates for its missing methylation cycle.
abstract: All organisms require S-adenosylmethionine (SAM) as a methyl group donor and cofactor for various biologically important processes. However, certain obligate intracellular parasitic bacteria and also the amoeba symbiont Amoebophilus asiaticus have lost the capacity to synthesize this cofactor and hence rely on its uptake from host cells. Genome analyses revealed that A. asiaticus encodes a putative SAM transporter. The corresponding protein was functionally characterized in Escherichia coli: import studies demonstrated that it is specific for SAM and S-adenosylhomocysteine (SAH), the end product of methylation. SAM transport activity was shown to be highly dependent on the presence of a membrane potential, and by targeted analyses, we obtained direct evidence for a proton-driven SAM/SAH antiport mechanism. Sequence analyses suggest that SAM carriers from Rickettsiales might operate in a similar way, in contrast to chlamydial SAM transporters. SAM/SAH antiport is of high physiological importance, as it allows for compensation for the missing methylation cycle. The identification of a SAM transporter in A. asiaticus belonging to the Bacteroidetes phylum demonstrates that SAM transport is more widely spread than previously assumed and occurs in bacteria belonging to three different phyla (Proteobacteria, Chlamydiae, and Bacteroidetes).
authors.affiliation: Zelluläre Physiologie/Membrantransport, Technische Universität Kaiserslautern, Kaiserslautern, Germany.
authors.forename: Ilka
authors.initials: I
authors.lastname: Haferkamp
authors.forename: Thomas
authors.initials: T
authors.lastname: Penz
authors.forename: Melanie
authors.initials: M
authors.lastname: Geier
authors.forename: Michelle
authors.initials: M
authors.lastname: Ast
authors.forename: Tanja
authors.initials: T
authors.lastname: Mushak
authors.forename: Matthias
authors.initials: M
authors.lastname: Horn
authors.forename: Stephan
authors.initials: S
authors.lastname: Schmitz-Esser
mesh_terms: D017920:Antiporters / Q000235:genetics / Q000378:metabolism*; D041963:Bacteroidetes / Q000235:genetics / Q000378:metabolism*; D003001:Cloning, Molecular; D019295:Computational Biology; D004926:Escherichia coli / Q000235:genetics / Q000378:metabolism; D012435:S-Adenosylhomocysteine / Q000378:metabolism; D012436:S-Adenosylmethionine / Q000378:metabolism*; D013379:Substrate Specificity
chemical_list: D017920:Antiporters; D012436:S-Adenosylmethionine; D012435:S-Adenosylhomocysteine