14. golgi and vesicular traffic Flashcards
Golgi apparatus
- golgi conissts of a stack of membrane-bounded cisternae located b/w the endoplasmic reticulum and the cell surface - resembles a stake of hollow pancakes
- named after camillo golgi (1898)
- primary function - to modify and sort proteins for export to other organelles
- golgi is cells shipping department
golgi structure
bottom to top (vesicle traffic in 2 directions, transitional elements, no ribosomes)
Cis golgi network
cis cisternae
medial cisternae
trans cisternae
trans-golgi network (network of tubules , vesicles) sorting area
to: secretory vesicles (lysosomes, cell membrane)
each compartment (cis,medial,trans) has unique enzymes:
functional differentation of the golgi complex can be studied with the electron microscope with specific techniques that detect different enzymes:
- cis region is where sugars and proteins are phosphorylated
- the medial (middle) region have enzymes that remove carbohydrates that were added in the ER, and then add new carbohydrates
- the trans region is the area where the lysosomes are sorted. therefore, it is rich in acid phosphatase, an enzyme found in lysosome
golgi function
stepwise glycosylation of proteins (adding sugars):
the making of glycoproteins:
-N-linked glycoproteins
-Continued processing of N-linked sugars (from ER)
-O-linked glycoproteins
N-linked glycoproteins
sugars attached at the N of the asparagine amino acid (also occurs in the ER)
Continued processing of N-linked sugars (from ER)
- substrate is a sugar-nucleotide (activated sugar), which is transported into the Golgi from cytoplasm
- Series of glycosyltransferases, which act in a rigidly determined sequence
O-linked glycoproteins
-sugars attached to the O of serine of threonine amino acids
only occurs in golgi
why glycosylate proteins ?
- makes proteins more hydrophilic
- allows them to bind to the extracellular matrix
- reduces their susceptibility to proteases(partly b/c carbohydrates are more rigid and thereby can provide physical barriers to the core of the protein)
- provides another level of structural complexity to proteins, which can:
- give the protein a new function
- provide more specificity in its interactions with other proteins
where do we see an abundance of glycosylated proteins ?
on the cell’s surface, where cell to cell interactions occur
What did John hodgson say about glycoproteins?
almost without exception, whenever two or more living cells interact in a specific way, cell surface carbohydrates will be involved
Golgi is a transient structure which means its …
constantly changing
the following contents all move through the layers of cisternae:
- membranes including proteins, phosphoglycerides, other membrane lipids
- materials within vesicles including proteins being processed and the enzymes doing the processing
two models of transport through the golgi:
- cisternal maturation model
- vesicular transport model
cisternal maturation model
each cisterna is physically moved up the stack and changes composition.
evidence: there are some materials, including large molecules that are never seen in the vesicles
vesicular transport model
- materials in the interior and in the membrane are brought up to next cisternae by vesicle transport
evidence: vesicles were found to move both forward and backward (anterograde and retrograde), so they could maintain enzyme differences
movement of vesicles
- not random of mediated by diffusion, as people once though
- vesicles travel along microtubules
- anchored to microtubules by various proteins, including one called dynactin (only with dynein,not kinesin; kinesin attaches with help of other adaptor proteins)
- vesicles propelled along microtubules by motor proteins, such as dynein and kinesin
is the golgi a permanent organelle?
no
the golgi breaks up and disappears at the onset of mitosis
by telophase of mitosis, the golgi reappears
we are still uncertain how it is recreated
growing microtubules may help collect and align small vesicles which later fuse together and the plus end
vesicular traffic
- vesicles 60-100nm diameter, deliver soluble and membrane-bound molecules by fusion of membranes
- vesicles are covered by proteins
three functions of the vesicle’s integral and peripheral proteins:
- vesicle formation/budding
- selection of vesicle contents by receptor system
- recognition and fusing to target membrane
three types of vesicles
- COP II-coated vesicles
- COP I-coated vesicles
- Clathrin-coated vesicles
COP II-coated vesicles
forward movement from ER and b/w cisternae
-anterograde movement
COP I-coated vesicles
retrograde movement from TGN to golgi to RER
Clathrin-coated vesicles
TGN to lysosomes and endocytosis
vesicles selectively transfer the following materials through the cell:
- contents w/in vesicles
- bulk flow, non-specific contents eg. water
- specific materials (proteins, carbohydrates, lipids) concentrated within
- specific membrane proteins
Some requirements of vesicular transport
- selection of contents
- receptor mediated
- production of vesicles
- by protein coat
- movement of the vesicles
- movement, tethering/docking, membrane fusion
selection of vesicle contents
-this requires receptors to select and carry specific contents
receptors are:
proteins, usually integral membrane proteins
bind other molecules (ligands) with very high specificity and very high affinity
-specificity and affinity give receptors the power to concentrate specific molecules
selection of vesicle contents by receptors
the coating of proteins inside a vesicle is the same as the inside of the target compartment and to the outside of the cell
ex. page 17
function of protein coats on vesicle exterior
- interact with the cytoplasmic tails of receptors
- ER export signals
- interact with other trans membrane proteins
- put mechanical force on membrane to form vesicles
three steps of protein coats on vesicle exterior
- when coat proteins assemble at the membrane, they force the lipid bilayer to ben
- coat proteins help gather the receptors that will carry cargo inside the vesicle
- shortly after the vesicle pinches off, the coat falls off and the vesicle is ready to travel to destination
COP II coated vesicles
page 21
coat contains several separate proteins
mechanisms of action:
1.receptors bind selected proteins within the lumen of the RER
* proteins destined for secretion
*ER resident proteins such as BIP are left behind
2. Sar1, a GTP binding protein, if active promotes assembly of the coat
3.coat proteins bind to cytosol side of specific receptors in RER membrane
4. coated bud forms, leaves the RER
*taking along receptor/cargo complexes
*also some membrane proteins (eg.. golgi glycosyltransferases)
*also membrane proteins involved in docking/fusion: destination tags-Rab and SNARES -not shown)
5.Sar1 hydrolyzes the GTP to GDP becomes inactive
6. disassembly of the coat
*things in the lumen of the vesicle and in the membrane stay there
7. the vesicle is now able to fuse with the cis golgi
COP 1-coated vesicles
-like COP II but the associated GTP binding protein is called ARF1
-retrograde movement
*trans to cis Golgi
*cis Golgi back to ERGIC, ER
COP I vesicles retrieve proteins-even though the COP II coated vesicles have used receptors to take desired protein from the ER and to allow the ER to retain most of its enzymes, there is some loss, due to bulk flow (ie a little unwanted extra protein gets dragged into the budding vesicle), and it needs to be sent back to the ER- these proteins will have retrieval signals to allow them to be returned
COP I coated vesicles have specific retrieval signals
- recognized by the receptors in the COP I membranes as they form:
- soluble proteins (eg protein disulfide isomerase and BiP) have a retrieval signal sequence Lys-Asp-Glu-Leu or “KDEL” that is bound to a KDEL receptor
- membrane proteins (Eg KDEL receptor) have a signal KKXX (Lys-Lys-?-?) at the protein C-terminal
- the KKXX retrieval signal binds to COP I proteins
- return to ER
