Getting to the cell surface Flashcards
Describe organelles present in the secretory pathway
Endoplasmic reticulum
Gogli apparatus
Endosomes
Vesicles
Are all proteins trafficked through the secretory pathway exported?
No
What proteins are exported?
Hormones
Enzymes
Extracellular matrix proteins
What proteins aren’t exported?
Incorporated into the membrane
Channels and transporters
Receptors
Cell adhesion molecules
Where are proteins synthesised?
Ribosomes of the rough endoplasmic reticulum
What are signal peptides?
Trigger transfer 15-20 amino acids long At the N-terminal end Hydrophobic Cleaved upon transit across the membrane
Where are signal peptides for multi-pass membrane spanning proteins?
Internal
Not cleaved
How do proteins cross the endoplasmic reticulum membrane?
Signal recognition particle recognises the signal peptide
Signal recognition particle binds to the signal peptide, pausing translation
Signal recognition particle binds to receptor in the membrane
Signal peptide is inserted into the translator
Translation continues and polypeptide chain is through the membrane
Signal recognition particle and receptor dissociate
The signal peptide is cleaved and protein is released into the ER lumen
What cleaves the signal peptide?
Signal peptidase
What are the main events that happen in the endoplasmic reticulum?
Signal peptide is cleaved Protein is glycosylated Protein is folded Disulfide bonds form Multimerisation can occur
What is N-glycosylation?
Most soluble and membrane spanning proteins are glycosylated
A preformed oligosaccharide (14 sugars) is transferred from dolichol to the new protein
What catalyses N-glycosylation?
Oligosaccharyl transferase
How do proteins leave the endoplasmic reticulum?
Vesicles containing soluble/membrane bound proteins bud from the ER and transit to the cis face of the Golgi
Describe the Golgi body
Comprises of flattened membrane compartments called cisternae
Each Golgi stack has two faces, the cis face and trans face
Two networks: the cis Golgi network and the trans Golgi network
Proteins arrive in vesicles at the cis face and leave in vesicles from the trans face
What are the two types of N-linked oligosaccharides?
Complex oligosaccharides
High mannose oligosaccharides
What are complex oligosaccharides?
Generated when the original N-linked oligosaccharide is trimmed and further sugars added
What are high mannose oligosaccharides?
Original N-linked oligosaccharide is trimmed and further mannose residues added
What is O-glycosylation?
Occurs when sugar residues are added to serine or threonine
Added by glycosyl transferases
What are proteoglycans?
Contain glycosaminoglycan chains
Unbranched polysaccharide chains composed of repeating disaccharide units
Special link tetrasaccharide is added to serine
What is the constitutive secretory pathway?
Operates continuously
Default pathway
Supplies the plasma membrane with newly synthesised membrane spanning proteins
Soluble proteins are continually secreted from the cell via this pathway
What is the regulated secretory pathway?
Some proteins are sorted into secretory granules where they are stored until required
Release is triggered by an extracellular signal
Vesicle fuses with plasma membrane and releases its content
What is the purpose of protein glycosylation?
Promotes protein folding
Aid cell recognition processess
Important in inflammatory response
Protect the cell
How are enzymes transported to lysosomes?
Lysosomal enzymes enter the secretory pathway and transported to lysosomes in vesicles
Their sorting signal is mannose-6-phosphate
How are proteins transported to the nucleus?
Nuclear localisation signals direct proteins to the nucleus
Runs of 5 or more basic amino acids anywhere in the protein sequence
