Soluble ER-resident proteins contain C-terminal KDEL sorting signal that allows them to be retrieved. 

TLS Online TPP Program

#Id: 7935

#Unit 2. Cellular Organization

The SRP is a cytosolic ribonucleoprotein particle that transiently binds to both the ER signal sequence in a nascent protein and the large (60S) ribosomal subunit, forming a large complex.

TLS Online TPP Program

#Id: 7936

#Unit 2. Cellular Organization

The SRP is made up of six proteins bound to a 300-nucleotide RNA, which acts as a scaffold for the hexamer.

TLS Online TPP Program

#Id: 7937

#Unit 2. Cellular Organization

Once the ER signal sequence emerges from the ribosome, it is bound by a signal recognition particle (SRP)

TLS Online TPP Program

#Id: 7938

#Unit 2. Cellular Organization

The SRP and the nascent polypeptide chain–ribosome complex bind to the SRP receptor in the ER membrane. This interaction is strengthened by the binding of GTP to both the SRP and its receptor.

TLS Online TPP Program

#Id: 7939

#Unit 2. Cellular Organization

The GTPase cycle analogous to the GTP hydrolysis coupled to the initiation of translation on the ribosome.

TLS Online TPP Program

#Id: 7940

#Unit 2. Cellular Organization

Once the SRP and its receptor have targeted a ribosome synthesizing a secretory protein to the ER membrane, the ribosome and nascent polypeptide chain are rapidly transferred to the translocon.