The signal-recognition particle (SRP) of one of the most acidophilic and hyperthermophilic archaeal cells, Acidianus ambivalens, and its putative receptor component, FtsY (prokaryotic SRP receptor), were investigated in detail. A. ambivalens Ffh (fifty-four-homologous protein) was shown to be a soluble protein with strong affinity to membranes. In its membrane-residing form, Ffh was extracted from plasma membranes with chaotropic agents like urea, but not with agents diminishing electrostatic interactions. Using unilamellar tetraether phospholipid vesicles, both Ffh and FtsY associate independently from each other in the absence of other factors, suggesting an equilibrium of soluble and membrane-bound protein forms under in vivo conditions. The Ffh protein precipitated from cytosolic cell supernatants with anti-Ffh antibodies, together with an 7 S-alike SRP–RNA, suggesting a stable core ribonucleoprotein composed of both components under native conditions. The SRP RNA of A. ambivalens depicted a size of about 309 nucleotides like the SRP RNA of the related organism Sulfolobus acidocaldarius. A stable heterodimeric complex composed of Ffh and FtsY was absent in cytosolic super-natants, indicating a transiently formed complex during archaeal SRP targeting. The FtsY protein precipitated in cytosolic super-natants with anti-FtsY antisera as a homomeric protein lacking accessory protein components. However, under in vitro conditions, recombinantly generated Ffh and FtsY associate in a nucleotide-independent manner, supporting a structural receptor model with two interacting apoproteins.

Abbreviations used: Biotin-NHS-ester, d-biotinoyl-6-aminohexanoic acid N-hydroxysuccinimide ester; Ffh, fifty-four-homologous protein; FtsY, prokaryotic SRP (signal-recognition particle) receptor; p[NH]ppG, guanosine 5′-[β,γ-imido]triphosphate; SR, SRP receptor; TBE, Tris/borate/EDTA buffer; TBS, Tris-buffered saline; TUVs, tetraether phospholipid unilamellar vesicles.

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