Signal recognition particles

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signal recognition particle 9kDa
Properties of human protein
Identifier
Gene name SRP9
External IDs

signal recognition particle 14kDa
Properties of human protein
Identifier
Gene name SRP14
External IDs

signal recognition particle 19kDa
Properties of human protein
Identifier
Gene name SRP19
External IDs

signal recognition particle 54kDa
Properties of human protein
Identifier
Gene name SRP54
External IDs

signal recognition particle 68kDa
Properties of human protein
Identifier
Gene name SRP68
External IDs

signal recognition particle 72kDa
Properties of human protein
Identifier
Gene name SRP72
External IDs

The signal recognition particle ( English particle recognition signal , SRP ) is a ribonucleoprotein that the co-translational transport of proteins into the endoplasmic reticulum (ER) of eukaryotes and the plasma membrane of prokaryotes is involved.

The core of the SRP is universal and conserved in all six biological realms .

construction

Components

SRP is composed differently in eukaryotes and prokaryotes , but has some similarities. For eukaryotes, mammalian SRP is best studied. There it consists of a 300 nucleotide- long, mostly double-stranded 7SL-RNA and six polypeptides with a mass of 9, 14, 19, 54, 68 and 72  kDa . They are therefore also referred to as SRP9, SRP14, SRP19, SRP54, SRP68 and SRP72. In the baker's yeast Saccharomyces cerevisiae , a unicellular eukaryote, the RNA (scR1) has a sedimentation coefficient of 11S. The proteins are homologous to those in mammals and are named Srp21p, Srp14p, Sec65p, Srp54p, Srp68p and Spr72p. Other primitive eukaryotes, such as the protozoa Giardia intestinalis or Trypanosoma cruzi , lack the homologues to SRP9 and SRP14. The parasite Encephalitozoon cuniculi also lacks a homologue to SRP68 and SRP72.

Prokaryotes have a much smaller SRP. In some gram-positive bacteria such as Bacillus subtilis , the RNA (scRNA) is 6S large, while the sedimentation coefficient of the SRP-RNA (ffs) in the gram-negative bacterium Escherichia coli is 4.5S and is 114 nucleotides in length. Bacteria have only one homolog to SRP54, which is referred to as Ffh ( fiftyfour homolog , English for “54 homolog”). In B. subtilis a 10 kDa protein, HBSu, also associates with the SRP RNA.

In archaea , the 7S-RNA is similar to that of eukaryotes, but these have only two homologues of SRP proteins: SRP19 and SRP54. In addition to a cytosolic SRP, plants also have an SRP in the chloroplasts . This consists of a SRP54-homologous protein (cpSRP54) and cpSRP43, a 43 kDa protein. Plastid SRP is unique in that it has no RNA.

structure

SRP consists of an S and an Alu domain, which reflect the two different functions of the complex. In eukaryotes, the S domain is composed of SRP54, SRP19, the heterodimer SRP68 / SRP72, and three helices of the SRP-RNA (helix 6-8). Only the helix 8 and the SRP54 homolog Ffh are conserved in bacteria, while the homologues SRP54 and SRP19 are preserved in archaea. There, however, the helix 7 is missing. The S domain mediates the binding to the signal sequence of a protein and to the SRP receptor on the membrane of the ER.

In eukaryotes, the Alu domain is composed of the heterodimer SRP9 / 14 and the remaining SRP-RNA. The 5 'and 3' ends of the RNA are also found there. The Alu domain is responsible for a delay during translation . Since this domain is absent in many bacteria, such a translation delay is unlikely there. In addition, data from cryoelectron microscopy indicate that SRP is bound to the small ribosomal subunit (40S) through the Alu domain.

Structure of the SRP-RNA in humans.

meaning

The complex has GTPase activity. The signal peptide , which consists of at least 8 non-polar amino acids in the center , is recognized in the GTP-bound state. When it leaves the ribosomal channel, it is recognized by the SRP54 subunit.

The SRP complex is present in eukaryotes in the cytosol. It binds reversibly to the signal sequence of a protein that has just been translated and to the large subunit of the ribosome . The signal sequence has a length of 15 to 50 amino acids and consists of a positively charged N-terminal region and a polar C-terminal region.

The translation of the protein is delayed and the entire complex of polypeptide, SRP and ribosome binds to an SRP receptor in the endoplasmic reticulum. The amino acid chain already present is transferred into the lumen of the ER via a translocation apparatus (translocon) . The SRP and its receptor hydrolyze GTP to GDP and dissociate. The translation will then continue.

history

The SRP was identified and characterized by Peter Walter when he was working as a PhD student under Günter Blobel .

literature

Web links

Individual evidence

  1. ↑ a b Pool, MR. (2005): Signal recognition particles in chloroplasts, bacteria, yeast and mammals . In: Mol Membr Biol . 22 (1-2); 3-15; PMID 16092520 ; doi: 10.1080 / 09687860400026348 .
  2. ^ Siegel, V. and Walter, P. (1985): Elongation arrest is not a prerequisite for secretory protein translocation across the microsomal membrane . In: J Cell Biol . 100 (6): 1913-1921; PMID 2581979 ; PDF (free full text access).
  3. ↑ Raine, A. et al . (2003): Targeting and insertion of heterologous membrane proteins in E. coli . In: Biochimie 85 (7); 659-668; PMID 14505821 ; doi: 10.1016 / S0300-9084 (03) 00130-5 .
  4. ↑ Halic, M. et al . (2004): Structure of the signal recognition particle interacting with the elongation-arrested ribosome . In: Nature 427 (6977); 808-814; PMID 14985753 ; doi: 10.1038 / nature02342 .
  5. ↑ Query at www.ncbi.nlm.nih.gov .
  6. ↑ Martoglio, B. and Dobberstein, B. (1998): Signal sequences: more than just greasy peptides . In: Trends Cell Biol . 1998 Oct; 8 (10): 410-415; PMID 9789330 ; doi: 10.1016 / S0962-8924 (98) 01360-9 .
  7. ↑ Isolating SRP .