Structural Biochemistry/The Signal Recognition Particle System in E. coli
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:
|In vitro||In vivo|
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.