Golgi Flashcards
what is the structure of the Golgi?
is a stack of flattened membrane bound compartments (cisternae)
CIs Golgi Network— Cis cisterna—Medial cisterna—Trans cisterna—Trans Golgi Network
where does the cis face of the Golgi lie? what does it do?
nearest the ER and is the site where vesicles from ER dock
what does the trans face do?
vesicles depart from here for the cell surface or other compartments
what is the function of ER exit sites?
-proteins accumulate here, and are sorted into vesicles to be transported into the Golgi
what must have happened to the protein for it to be exported to the Golgi?
- they MUST BE FULLY FOLDED before leaving the ER
- if folded incorrectly, then undergo proteasomal degradation
- if fully and CORRECTLY folded then they will be exported to Golgi
how do vesicles transport proteins from the donor to the recipient?
1- BUDDING from specialised coated regions of donor compartment
2- SEPARATION from donor compartment and movement to the recipient compartment
3- FUSION (membrane fusion) w/ recipient compartment
4- contents of vesicle released into recipient
why are vesicles needed to transport proteins?
- proteins can be hydrophobic or hydrophilic
- for protection during transport
- keeps the protein in correct environment
how do the proteins reach the vesicles needed for transport?
- transmembranous receptors in SPECIFIC AREAS of ER memb bind w/ cargo
- O the correct cargo proteins are concentrated in one area
- a signal (exit signal) is produced by each protein and is recognised by a specific receptor
give an example of an exit signal going wrong
ERGIC53 receptor binds to mannose on proteins (Factors V and VIII) that are blood clotting factors
- if there is ERGIC53 deletion it can lead to haemophilia
- O no export of factors V and VIII to Golgi O no export to blood
what causes vesicles to bud off?
-coat protein COPII interacts with the tail of the receptor (in the cytosol), causing vesicle to bud off
what happens after the vesicle has budded off? how does it move to the Golgi?
- once the vesicle has budded off from ER, they rapidly lose their COPII coat
- then vesicles fuse- HOMOTYPIC FUSION- like joins w/ like
- so many fuse that it forms VESICULAR TUBULAR CLUSTER (VTC)
- moves ALONG MICROTUBULES to Golgi
- cargo released from receptor by DECREASE IN pH (deforms receptor)
- now cargo is part of the Golgi
how are the receptors that transported the cargo proteins recycled?
- COPI coats parts of the VTC/Golgi
- vesicles bud from the Vesicular Tubular Cluster & Golgi
- vesicles have COPI coat removed and move back to ER
other than the receptors that transported cargo to the Golgi, what else may need to be returned?
proteins that escaped the ER (via vesicles) by mistake
how are proteins that were meant to stay in the ER but got transported to Golgi moved back to the ER?
- such proteins contain -KKXX amino acid chain, which interacts directly w/ COPI
- causes return of protein to ER
how are soluble ER proteins returned to ER if they have been accidentally transported to the Golgi?
they also contain retrieval signals- the amino acid chain KDEL, which interacts w/ COPI
-causes return of protein to ER
in terms of oligosaccharide processing, what happens in the Golgi?
trimming of sugars, addition of sugars (by localisation of enzymes)
O the protein that enters the Golgi is DIFFERENT when it leaves the Golgi
what is o-linked glycosylation? where does it occur?
what catalyses the reaction?
- occurs in Golgi (cisternae)
- addition of sugars to -OH group of Ser, Thr or hydroxylysine
- catalysed by GLUCOSYL TRANSFERASES
give one difference between O-linked glycosylation and N-linked?
- N-linked is the addition of a 14 carbohydrate group at once, O-linked is one at a time
- O-linked is addition of sugars onto O not N
what other modifications can occur within the Golgi?
- phosphorylation of oligosaccharides on lysosomal proteins
- sulphation of tyrosine and carbohydrates (inc -ve charge)
why glycosylate?
- PROTEIN MARKER for: complete folding, targeting transport between ER and Golgi, sorting within Golgi
- PROTECTION, oligosaccharides prevent proteases approaching and digesting the protein
- CELLULAR MARKER, cell-cell recognition and adhesion
- REGULATION, if cell surface receptors are glycosylated, altering their glycosylation can change their activity
what does COPII do?
COPI?
Clathrin?
COPII- coats vesicles for transport ER—> Golgi
COPI- coats vesicles for transport Golgi—>ER
Clathrin- coats vesicles for transport between Golgi, lysosomes and plasma membrane
what is the structure of clathrin?
3 large and 3 small polypeptide chains, which form a 3 legged triskelion
what is adaptin?
what is it needed for?
-needed to attach the CLATHRIN coat to the membrane
how many different types of adaptin are there?
at least 4, each recognising a different set of cargo receptors
