Structural Biochemistry/The Signal Recognition Particle System in E. coli

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The signal recognition particle (SRP) system, which contains the SRP and its receptor, plays a major role in targeting of ribosomes and IMP-encoding transcripts to the endoplasmic reticulum of eukaryotes and the cytoplasmic membrane of bacteria.

There are 3 essential components of the SRP system in E. coli:

  • The SRP protein – Ffh
  • The SRP receptor – FtsY
  • A small stable 4.5S RNA

The roles of SRP system in E. coli is related to the biosynthesis of IMPs:

  • SPR-mediated ribosome targeting (S-MRT)
  • SRP receptor-mediated ribosome targeting (SR-MRT)

1. S-MRT is based mainly on in vitro studies with purified components, has 2 stages:

 SRP binds to cytosolic mRNA-ribosome nascent chain complexes.
 The SRP-RNC complexes targeted to the membrane-bound FtsY.

2. SR-MRT is based mainly on in vivo studies, has 3 stages:

 During FtsY translation, FtsY and ribosomes are targeted to the membrane in an SRP-independent manner.
 IMP-encoding mRNAs are targeted to the membrane-bound ribosomes.
 SRP interacts with a hydrophobic nascent polypeptide, the ribosomes, and the SRP receptor.

Both of the models require RNCs, SRP, and SRP receptor to form a complex on the membrane, transfer RNC to the translocon. However, SR-MRT differs from S-MRT by the additional targeting stages:

Ribosome targeting:

In vitro In vivo
  • The E. coli SRP can interact with ribosomes and RNCs in aqueous solution and in the cytoplasm.
  • The ribosome-SRP interaction does not rule out the possibility that the same interaction may occur between membrane-associated SRP and membrane-bound ribosomes translating IMPs.
  • Preformed soluble RNC-SRP complexes can target the membrane-bound SRP receptor.
  • The E. coli SRP can interact with ribosomes and RNCs in aqueous solution and in the cytoplasm.
  • The interactions between the SRP and cytosolic ribosome play role as a quality control mechanism to prevent premature synthesis of IMPs in the E. coli cytosol.
  • SRP may not be required for ribosome targeting.

mRNA targeting:

IMP-encoding transcripts in E. coli are targeted to the membrane in a translation-independent manner.

The uracil-bias is an evolutionarily ancient feature of IMP-encoding mRNAs.

Uracil-richness is required for mRNA targeting to the membrane.