%0 Journal Article
%T Archaea Signal Recognition Particle Shows the Way
%A Christian Zwieb
%A Shakhawat Bhuiyan
%J Archaea
%D 2010
%I Hindawi Publishing Corporation
%R 10.1155/2010/485051
%X 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. 1. Introduction Protein sorting fundamentally maintains the identity and function of every cell with participation of the signal recognition particle (SRP). SRP components have been found in nearly all organisms [1]. Except in chloroplasts, SRP is a ribonucleoprotein [2]. The SRP RNA is typically composed of about 300 nucleotide residues and forms a complex with an extraordinarily conserved protein named SRP54 in archaea and eukarya or Ffh (fifty-four homolog) in the bacteria. A 19£¿kDa protein, SRP19, is present in archaea and eukarya, but absent in the bacteria. Polypeptides which are homologous to the eukaryal SRP9/14 and SRP68/72 heterodimers have not been found in the archaea genome sequences giving rise to an archaea SRP which is dominated by RNA [3, 4]. SRP interacts with secretory signal or membrane-anchor sequences upon their emergence from the ribosomal exit tunnel. In vitro and in vivo experiments carried out in eukaryotic protein sorting systems have shown that the SRP delays or blocks the translation of the to-be-targeted polypeptides. Translation resumes when the SRP-bound ribosome nascent chain complex (RNC) binds to the membrane-associated FtsY (filamentous temperature sensitive Y) or, in eukaryotes, the alpha subunit of the SRP receptor (SR ). The interaction between SRP54 and the SR increases the affinity of
%U http://www.hindawi.com/journals/archaea/2010/485051/