Getting to the cell surface Flashcards
What is the process of translation for many proteins?
mRNA is translated into protein on ribosomes in the cytoplasm.
Proteins then either remain in the cytoplasm or are targeted to the nucleus, mitochondria, peroxisomes or chloroplasts by localisation signal. e.g. the nuclear localisation signal.
What are the two classes of proteins targeted to the cell membrane?
Secreted / transmembrane proteins - like hormones and growth factors.
Membrane proteins that are inserted into the cell membrane for different functions.
What are the functions of membrane proteins?
Receptors for hormone and growth factors.
Cell adhesion molecules
Ion channels
Light perception - rhodopsin.
How do translated proteins get into the ER lumen?
Dedicated pores in the ER membrane moves proteins through the membrane.
How do proteins get into the lumen of the ER - signal peptide?
mRNA associates with ribosomes, translation starts.
As soon as signal peptide is translated, it is recognised by a protein RNA complex called the signal recognition particle SRP.
This targets the translating protein and ribosome to the membrane.
How do proteins get into the lumen of the ER - continuing translation?
The ribosome is targeted to pores in the membrane called the translocator.
The signal peptide is cleaved off and the rest of the protein is threaded through the pore into the lumen of the ER by co-translational translocation.
How do proteins get into the lumen of the ER - terminating translation?
When the ribosome encounters a stop codon on the mRNA, translation is terminated.
The ribosome dissociates from the membrane and the protein is now fully loaded into the ER lumen.
The ribosome subunits are free to bind under more mRNAs and start another round of translation.
What is the binding site on the SRP?
The SRP also has a binding site for the ribosome, and it causes a transient pause in translation.
This pause lasts until the SRP is recognised by a protein located in the membrane of the SRP receptor.
What happens when the SRP receptor binds to SRP?
The SRP receptor binds the SRP and brings the whole complex, including the ribosome and peptide to the ER, where it is now tethered next to pores in the membrane formed by the protein translocator.
The signal peptide and ribosome are transferred from the SRP to the protein translocator.
What does the SRP receptor do once transferred to the translocator?
The complex between the signal recognition particle and its receptor and the ribosome now dissociates and breaks apart and the SRP and SRP receptor are free to participate in additional rounds of recruitment.
What does the ribosome do after it dissociates from the SRP complex?
The ribosome remains associated with the translocator and is free to continue translation, threading the growing polypeptide through the translocator.
Eventually the fully translated protein will pass entirely through the translocator and be localised to the ER lumen.
What are the key components of co-translational translocation?
ribosomes
signal peptide
SRP
SRP receptor
Translocator
What is the signal peptide?
15-20 amino acid hydrophobic sequence.
Located at the N-terminal end of the secreted proteins, that is recognised by the signal recognition particle to allow targeting to the translocator.
Normally cleaved from proteins before the end of translation.
What is the SRP?
Signal peptide is recognise by the Signal recognition particle to allow targeting to the translocator.
Cycles between the cytoplasm where it catches the ribosomes and the ER membrane where it docks the signal peptide onto the translocator.
What is the SRP receptor?
Docking is enabled by the SRP receptor.
An ER membrane protein recognises the ribosome-SRP complex and localises it to the ER membrane and translocator.
