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Bhuiyan, Shakhawat (1)
Eichler, Jerry (1)
Hinck, Andrew P. (1)
Huang, Qiaojia (1)
Pakhomova, Olga N. (1)
Yin, Jiaming (1)
Year of Publication
Archaea Signal Recognition Particle Shows the Way
Archaea SRP is composed of an SRP RNA molecule and two bound proteins named SRP19 and SRP54. Regulated by the binding and hydrolysis of guanosine triphosphates, the RNA-bound SRP54 protein transiently associates not only with the hydrophobic signal sequence as it emerges from the ribosomal exit tunnel, but also interacts with the membrane-associated SRP receptor (FtsY). Comparative analyses of the archaea genomes and their SRP component sequences, combined with structural and biochemical data, support a prominent role of the SRP RNA in the assembly and function of the archaea SRP. The 5e motif, which in eukaryotes binds a 72 kilodalton protein, is preserved in most archaea SRP RNAs despite the lack of an archaea SRP72 homolog. The primary function of the 5e region may be to serve as a hinge, strategically positioned between the small and large SRP domain, allowing the elongated SRP to bind simultaneously to distant ribosomal sites. SRP19, required in eukaryotes for initiating SRP assembly, appears to play a subordinate role in the archaea SRP or may be defunct. The N-terminal A region and a novel C-terminal R region of the archaea SRP receptor (FtsY) are strikingly diverse or absent even among the members of a taxonomic subgroup.
The conserved adenosine in helix 6 of Archaeoglobus fulgidus signal recognition particle RNA initiates SRP assembly
Pakhomova, Olga N.
Hinck, Andrew P.
The signal recognition particle (SRP) RNA helix 6 of archaea and eukaryotes is essential for the binding of protein SRP19 and the assembly of a functional complex. The conserved adenosine at the third position of the tetraloop of helix 6 (A149) is crucial for the binding of protein SRP19 in the mammalian SRP. Here we investigated the significance of the equivalent adenosine residue at position 159 (A159) of Archaeoglobus fulgidus SRP RNA. The A159 of A. fulgidus and A149 of human SRP RNA were changed to C, G or U, and fragments containing helix 6 or helices 6 and 8 were synthesized by run-off transcription with T7 RNA polymerase. The ability of recombinant A. fulgidus and human SRP19 to form ribonucleoprotein complexes was measured in vitro. The simultaneous presence of A149 and helix 8 is required for the high-affinity binding of SRP19 to the human SRP RNA. In contrast, A. fulgidus SRP19 binds to the SRP RNA fragments with high affinity irrespective of the nature of the nucleotide, demonstrating that A159 does not directly participate in protein binding. Instead, as indicated by the resistance of the wild-type A. fulgidus RNA towards digestion by RNase A, this residue allows the formation of a tightly folded RNA molecule. The high affinity between A. fulgidus SRP19 and RNA molecules that contain both helices 6 and 8 suggests that A159 is likely to initiate archaeal SRP assembly by forming a conserved tertiary RNA– RNA interaction.
protein–RNA interactions; site-directed mutagenesis; tetraloop
Getting on target: The archaeal signal recognition particle
Protein translocation begins with the efficient targeting of secreted and membrane proteins to complexes embedded within the membrane. In Eukarya and Bacteria, this is achieved through the interaction of the signal recognition particle (SRP) with the nascent polypeptide chain. In Archaea, homologs of eukaryal and bacterial SRP-mediated translocation pathway components have been identified. Biochemical analysis has revealed that although the archaeal system incorporates various facets of the eukaryal and bacterial targeting systems, numerous aspects of the archaeal system are unique to this domain of life. Moreover, it is becoming increasingly clear that elucidation of the archaeal SRP pathway will provide answers to basic questions about protein targeting that cannot be obtained from examination of eukaryal or bacterial models. In this review, recent data regarding the molecular composition, functional behavior and evolutionary significance of the archaeal signal recognition particle pathway are discussed.
protein targeting; protein translocation; ribonucleoprotein complex; RNA; signal sequence
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