Protein trafficking Flashcards
What molecules within the cell are tagged with sugars, where are they tagged and what do these sugar tags do?
Proteins are tagged with glycolipids. They are carried on the cell membranes and provide PROTECTION FROM FOREIGN ELEMENTS.
What types of cells are rich in carbohydrates and why?
Epithelial cells as they need to protect themselves from the harsh external environment
Where are cytosolic proteins made? Where are nuclear proteins made? Where are membrane-bound proteins made?
Cytosolic - the cytosol
Nuclear - the outer nuclear membrane
Membrane bound / secretory proteins - the rough ER
What is a polyribosome and where are they found?
A polyribosome is a common pool of ribosomal sub units that is free in the cytosol and then assembles at the surface of the ER. The mRNA in these ribosomes will code for an ER-targeted protein
What occurs following protein synthesis at the ribosomes of the rough ER? (er>smooth>Golgi). Role of lipid anchors?
The proteins move to the smooth ER where lipids are produced and vesicles form, then final addition of sugars and sorting happens in the Golgi apparatus. Lipid anchors are used in the ER to keep proteins anchored to the membrane.
How are proteins sent to the right place?
A signal sequence - attached to mRNA and ensures a protein is transported to the correct place.
What is an SRP and what is its purpose?
Signal recognition particle - guides the ribosome from the cytosol to the ER membrane following its binding to the signal peptide attached to the free mRNA - therefore helps in translocation of the mRNA so that it can be translated
What is signal peptidase and how does it work
Signal peptidase is an ER-based enzyme and cleaves the signal sequence away from the ribosome once the protein of interest has been translated
CONSTITUATIVE and REGULATED pathways
Constituative - unregulated membrane secretion - proteins are released from vesicles as soon as the vesicles arrive at the membrane
Regulated - hormonally regulated - vesicles are released when the cell reeves the appropriate extracellular signal
Differential centrifugation - why is it used and how does it work ?
Separates light and heavy vesicles - microsomes are obtained by homogenisation (contain both rough - with ribosomes - and smooth microsomes), then span at a glucose gradient and light and heavy microsomes are separated.
Rough sediment at a high glucose concentration.
Can then conduct detailed protein synthesis.
ORGAN TRANSPLANTATION - why do glycolipids pose a problem?
Humans carry B-galactose and animals a-galactose. These are sugar modifications made to proteins. When humans are transplanted with animal organs, they produce antibodies against the a-galactose and cause immunal rejection of the organ.
PROTEIN TRIMMING - what is it and where does it occur? Insulin as an example?
Also known as protein processing or maturation, proteins are cleaved or trimmed prior to secretion. Insulin is an example of this - in the Golgi it is cleaved from proinsulin to insulin, and packaged into vesicles as insulin.
TYPE 1 DIABETES - involvement in protein trimming?
Proinsulin in type 1 is misfolded - functioning insulin is therefore not produced and antibodies are produced against pancreatic cells. Causes increase in blood glucose.
CARBOHYDRATE ADDITION - what is the point, where does it start, and what is the process?
Allows cell-cell recognition and protein stability/protection and cross-species separation (e.g. humans have b-galactose).
Starts in the ER - sugars added as ‘quality control tags’, then in the Golgi cisternae carbohydrate growth and trimming occurs in a step by step sequence in the golgi cisternae (relies on transport to each adjacent cisternae by vesicles, in a cis-trans direction).
Protein glycosylation in relation to the golgi?
Protein glycosylation is a multi step process as each new modification is made at the next cisternae in the golgi apparatus. Each cisternae keeps specific glycosylation enzymes away from each other.
