Signal recognition particle receptor

Miller, J. D., Tajima, S., Lauffer, L., and Walter, P. (1995). The beta subunit of the signal recognition particle receptor is a transmembrane GTPase that anchors the alpha subunit, a peripheral membrane GTPase, to the endoplasmic reticulum membrane. J. Cell Biol. 128, 273-282. 

Subunits of signal recognition particles and their receptors... 

Sickle cell disease 16, 359, 360 Side chains packing 75 Siderophilins 840 Siderophore(s) 838, 838s chelate effect 310 receptors 839 Sieve plates 30 Signal peptidase 522, 620 Signal recognition particle (SRP) 520 Signal sequences 519 Signaling 1... 

Import into the endoplasmic reticulum requires an N-terminal signal sequence that contains a long stretch of hydrophobic amino acids. The mitochondrial transit peptide is a hydrophilic sequence rich in serine and threonine, with regularly spaced basic amino acids. Import into the ER requires the signal recognition particle and its receptor, but mitochondrial import does not require the SRP and presumably uses a different receptor. Import into mitochondria requires a membrane potential, but import into the ER does not. 

The mRNA for the secretory protein binds to a free cytoplasmic ribosome and protein synthesis begins. The first part of the protein made is the N-terminal signal peptide. A signal recognition particle (SRP), which is a complex of a 7S RNA and six proteins, binds to the signal peptide and stops further protein synthesis. This stops the secretory protein from being released prematurely into the cytosol. The ribosome-mRNA-SRP complex now binds to an SRP receptor, a protein on the surface of the ER. The ER membrane also contains a ribosome receptor protein associated with a protein translocator. In a concerted series of reactions, the ribosome is held tightly by the ribosome receptor protein, the SRP... 

The process of protein export involves a small, cytoplasmic ribonucleoprotein particle (the Signal Recognition Particle or SRP) with the signal coding mRNA sequence and/or the signal peptide itself. This interaction stops translation of the protein. Then, the stalled or arrested ribosome moves to the endoplasmic reticulum (ER). A receptor on the ER binds the SRP. 

Figure 12.6 Roles of signal recognition particles (SRP), SRP receptor (docking protein), ribosome binding proteins (ribophorin I and II), and signal peptidase in translocation of nascent polypeptide chains across ER membrane. AA, amino acids.

Proteins are targeted to various locations after synthesis by signal sequences. Thus, proteins destined for the ER, the mitochondria and chloroplasts have particular kinds of signal sequences at the N-terminus. ER-targeted proteins enter the ER directly off rough ER ribosomes via a signal recognition particle (SRP) complex that is linked to an SRP receptor and a ribosome receptor-transmembrane peptide translocation complex associated with the ER membrane. Within, the ER polypeptides are processed and folded and S—S links are formed. 

SRIF-R, somatostatin (somatotropin release inhibiting factor) receptor SRP, signal recognition particle S-S, disulphide... 

As in the SR protein family (serine- and arginine-rich proteins) also sarcoplasmic reticnlnm, scavenger receptor Synchrotron radiation circnlar dichroism Semm response factor, a nbiqnitons transcription factor Signal recognition particle SR protein kinase... 

The structure of the mammalian signal recognition particle (SRP) receptor as prototype for the interaction of small GTPases with longin domains, J. Biol. Chem. 281, 8898-8906, 2006. 

Fig. 12.3. Current model for vectorial translation depicted for one of the subunits of acetylcholine receptor. The various events are described in the text. ER, endoplasmic reticulum SRP, signal recognition particle.

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