The archaeal signal recognition particle: steps toward membrane binding

Signal recognition particles and their receptors target ribosome nascent chain complexes of preproteins toward the protein translocation apparatus of the cell. The discovery of essential SRP components in the third urkingdom of the phylogenetic tree, the archaea (Woese, C. R., and Fox, G. E. (1977). Proc. Natl. Acad. Sci. U.S.A. 74, 5088-5090) raises questions concerning the structure and composition of the archaeal signal recognition particle as well as the functions that route nascent prepoly peptide chains to the membrane. Investigations of the archaeal SRP pathway could therefore identify novel aspects of this process not previously reported or unique to archaea when compared with the respective eukaryal and bacterial systems.

Novel Functional Aspects of the Membrane-Bound exo-Pyrophosphatase of the Hyperthermoacidophilic Archaeon Sulfolobus Are Provided by Analysis of Its Gene and the Adjacent Gene Cluster

The gene of the previously described plasma-membrane-bound acidic pyrophosphatase (exo-PPase) and adjacent genes of the hyperthermoacidophilic crenarchaeon Sulfolobus acidocaldarius (DSM 639) were cloned and sequenced. The 4-kb gene cluster comprises four open reading frames (sepp, simp, sabc, and satr) encoding the pyrophosphatase, a small hydrophobic protein of unknown cellular function, a hydrophilic ABC transport ATPase, and an amino transferase. The four proteins have deduced molecular masses of 21, 16, 34, and 48 kDa, respectively. Sepp, simp, and sabc are transcribed as monocistronic mRNAs from which sepp and sabc have been heterologously expressed by in vitro translation using reticulocyte lysates. The Sulfolobus acidocaldarius acidic exo-pyrophosphatase is a membrane-residing protein anchored with five transmembrane alpha-helices. Alignments with protein sequences from databases together with predictions of membrane topology reveal a novel group of proteins with the conserved phosphatase motif KxxxxxRP-(x12-54)-PSGH-(x31-54)-SRxxxxxHxxxD. For none of them a phosphatase or pyrophosphatase activity has yet been described except for the authentic Sulfolobus acidocaldarius protein. On the basis of these investigations a direct role of the exo-PPase in dolichyl phosphate or pyrophosphate hydrolysis and in resistance to the peptide antibiotic bacitracin is discussed.

Protein-protein, protein-RNA and protein-lipid interactions of signal-recognition particle components in the hyperthermoacidophilic archaeon Acidianus ambivalens

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 supernatants, indicating a transiently formed complex during archaeal SRP targeting. The FtsY protein precipitated in cytosolic supernatants 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.