Lecture 5 - Introduction on RER, Secretory Pathway and Insulin Flashcards
What types of proteins go through the secretory pathway?
Proteins translated at the RER include:
Golgi apparatus proteins
Lysosomal proteins
Plasma membrane proteins
Secreted proteins
(proteins targeted to cytoplasm and mitochondria do not, and are not translated in the RER)
Where is insulin translated, transported, and secreted?
Insulin is a signalling protein secreted only by the beta-cells in the pancreas
* Translated at the RER, transported via the COP Il pathway and eventually gets secreted out of beta-cell
* Regulating blood glucose levels such as after a heavy meal
What is the role of the signal peptide in the formation of insulin?
Signal peptide functions to target the nascent chain, together with the ribosome and the mRNA to the RER.
To enter the secretory pathway, nascent preproinsulin is delivered by the signal recognition particle (SRP) to the RER membrane, where preproinsulin is translocated to the luminal side of the ER and cleaved by signal peptidase, forming proinsulin.
What happens to proteins after entering the RER?
Post-translational Modifications of proteins in the RER
- Glycosylation
- Disulphide bonds
Purpose of such modifications
- increase stability of the proteins
- promote folding of proteins, association of proteins
Glycosylation:
- N-linked glycosylation when carbohydrate groups are added to the nitrogen group on the side-chain of Asparagine or arginine
- or O-linked glycosylation when the carbohydrate groups are added to the hydroxyl-oxygen of Serine or Threonine
When moving out of the RER, how does vesicular transport occur?
The proteins make use of the RER membrane to form vesicles that they use as vehicles for transport to the various destinations.
Several issues to consider:
* selection of proteins (cargoes) into the vesicles
* making the vesicles from the RER membrane
COP Il coat proteins
- forms on the RER membrane
- forces a curvature so the RER membrane buds out that eventually becomes a vesicle
- at the same time, cargoes are selected into the budding vesicle
What are the key events in COPII vesicle formation?
Sec12 is a GTP-exchange factor that is found at the RER membrane
Sar1 is a GTPase that inserts itself into the RER membrane upon GDP to GTP exchange
Sar1 forms a nucleation centre around which the COPII coats assemble on the RER membrane.
Sec24 interacts with RER membrane proteins with one part of the protein exposed to the cytoplasm. The membrane protein interacts with a soluble or lumenal protein, which is then included into the vesicle that is forming.
Pinching off
Uncoating of COPII coat - hydrolysis of GTP to GDP by Sar1.
What are the three distinct regions in a signal peptide?
SP in general are on average 16 to 30 amino acid residues in length comprising a characteristic tripartite structure:
(1) a hydrophilic, usually positively charged n-region,
(2) a central hydrophobic h-region of 5-15 residues and
(3) a c-region with the cleavage site for signal peptidase (SPase)
These are characteristics across different SP - so there is no exact SP sequences for all the different proteins targeted to the RER.
