Lec 10: Golgi Apparatus Flashcards
GA =
Golgi apparatus (for short hand)
GA is involved with
processing and packaging of protein products for secretion or passage to other organelles (cell membrane, ER, endosome, lysosome, secretory vesicle…)
GA Consists of
a series (3 – 8) of cisternae (flat, disc-like membranous sacs)
GA Series of cisternae aka
a Golgi Stack (dictyosome in plants)
Lumen of the Golgi cisternae aka
the intracisternal space
GA has how many sides?
2
GA 2 sides:
cis (or forming) face
trans (or maturing) face
(Cis Golgi)
Generally oriented towards the…
Receives…
Here, the…
…RER
…transition vesicles from the RER (contain lipid or proteins synthesized in the ER)
… initial Golgi cisternae are also known as the cis-Golgi network (CGN)
(Trans Golgi)
Generally oriented away from the…
Aka…
From it…
…RER
…the trans-Golgi network (TGN)
…transport vesicles carry processed proteins and lipids to: secretory granules, endosomes, lysosomes, plasma membrane…
Whats between cis and trans Golgi?
Medial cisternae are all of the Golgi cisternae between the cis and trans faces.
- What does it mean that the Golgi is compartmentalized?
- All processing is associated with…
- Primarily…
- The complement of processing enzymes in the intracisternal space of the Golgi are specific to different regions of the Golgi (from the CGN to the TGN) (and the ER as well).
- post-translational modification
- glycosylation, modification of glycosylation, and phosphorylation of specific proteins
2 models may describe the flow of material through the Golgi Complex:
- ) Stationary Model
2. ) Maturation Model
(Stationary Model)
- Each layer (cisterna) is…
- _____ _____ carry the contents from one layer (cisterna) to the next
- stationary and stable.
- Shuttle vesicles
(Maturation Model)
- Each cisterna is only…
- Each cisterna moves…
- Each cisterna originates initially as…
- Each cisterna eventually…
- transiently positioned in one layer.
- from one layer to the next
- the CGN then as more transition vesicles fuse above it, it becomes part of the medial cisternae
- becomes the TGN which produces the transport vesicles
- Is it also possible (probable?) that a combination of the two models exist?
- However, the most recent data supports which model?
- Yes
- Maturation model
- If the maturation model is true, then how do the processing enzymes of the CGN stay in the appropriate cisternae?
- What else has been suggested to exist? where? what does it do?
- Retrograde transport back to the CGN from the medial cisternae via shuttle vesicles
- In addition, an endoplasmic reticulum-golgi intermediate compartment/complex (ERGIC) has been suggested to exist, between the ER and the golgi.
It is a site of anterograde and retrograde sorting. The ERGIC also contributes to the concentration, folding, and quality control of newly synthesized proteins
(Golgi Function)
- Most materials move from…
- Known as…
- the CGN towards the TGN
- Anterograde (forward) transport
(Golgi Function)
- Some materials can move from…
- Known as…
- Allows for…
- Assures that…
- Occurs via…
- Example:
- the TGN towards the CGN
- Retrograde transport
- recycling of internal components
- each level of the Golgi (and ER) maintains specific compartments with specific proteins/enzymes
- specific protein “sequence tags” (amino acid signal peptide) that designate the proteins for return.
- Ex: Retrieval tag “–KDEL–” (-lys-asp-glu-leu-) designates the protein for retrieval back to the ER.
(Golgi Function)
- Retention of proteins in the Golgi is…
- Likely some…
- less well understood
- specific protein sequence tags
(Golgi Function)
1.) Also appears to involve the…
- ) All known Golgi-specific proteins are…
- ) Length of the transmembrane domain relates to…
- ) Membrane thickness increases from…
- ) Correlation between the…
- Golgi proteins with shorter transmembrane domains are found towards the…
- Golgi proteins with longer transmembrane domains are found in the… - ) If you artificially lengthen the transmembrane domain of a CGN Golgi protein, it will…
- ) length of the transmembrane domains of the proteins
- ) integral membrane proteins (many with just a single transmembrane domain)
- ) the region (or cisterna) of the Golgi in which the protein resides
- ) ~5 nm to ~7 nm from CGN to TGN
- ) length of the transmembrane domain, Golgi membrane thickness, level at which the proteins are found.
- CGN
- TGN - ) appear towards the TGN and even in the plasma membrane (which is ~8 nm thick)
(Transmembrane Domain Length) ~5 nm = ~6 nm = ~7 nm = ~8 nm =
~5 nm = CGN
~6 nm = Medial Cisternae
~7 nm = TGN
~8 nm = Plasma membrane
Transport vesicles =
Vesicles that form from the TGN (transport vesicles)
Three main fates (3 main types of transport vesicles)
1.) Give rise to secretory vesicles (constitutive or regulated)
- ) Give rise to early endosomes, contain lysosomal hydrolases – digestive enzymes (but in an inactive state, at this point)
- 1st step in formation of Lysosome
3.)Retrograde transport
Lysosome functions as a
digestive organelle
Lysosome formation steps: (5)
- ) Fusion of specific transport vesicles (contain lysosomal hydrolases - inactive) & endocytic vesicles to form early endosomes.
- ) The early endosomes mature into late endosomes (has a full complement of inactive digestive enzymes)
- ) Later it matures into a lysosome (digestive enzymes are activated, due to a lowering of pH in lumen of lysosome).
- ) Lysosomal acidification (reduced pH) is achieved by the activity of a v-type proton pump, that pumps protons (H+) from the cytosol into the lumen of the lysosome.
- ) Existing lysosomes may also fuse with additional late endosomes.
Golgi (& RER) Functions: (2)
- ) Protein Modification
2. ) Protein Sorting
(Golgi (& RER) Functions/Protein Modification)
- In the RER: (1)
- In the Golgi: (3)
1.) Initial glycosylation (mainly N-linked to asparagine)
- ) Further glycosylation (mainly O-linked to serine and threonine)
- ) Modification of both N- and O-linked glycosylation
- Hundreds of different glucan synthases (synthesize oligosaccharides) and glycosyl transferases (transfer carbohydrates to proteins) are present in the Golgi (and the ER). - ) Protein tyrosine sulfation
- Sulfate groups are added to some tyrosine residues
- Sulfated tyrosines are generally found on either secreted proteins or extracellular portions of plasma membrane proteins
- Plays a role in protein-protein interactions
(Golgi (& RER) Functions/Protein Modification)
(In the Golgi: Protein tyrosine sulfation)
- Sulfate groups are…
- Sulfated tyrosines are…
- Plays a role in…
- added to some tyrosine residues
- generally found on either secreted proteins or extracellular portions of plasma membrane proteins
- protein-protein interactions
(Golgi (& RER) Functions/Protein Sorting)
- Various proteins must be…
- Involve…
- &…
Example:
- designated for specific cellular compartments
- specific sequence tags or specific glycosylation
- Transmembrane domain length
Ex: Targeting of soluble proteins for lysosomes
- In part, involves a mannose-6-phosphate residue in the glycosylated portion of a protein
(Protein Sorting/Secretory Pathways): (2)
- ) Constitutive Secretion
2. ) Regulated Secretion
(Protein Sorting/Secretory Pathways)
(1. Constitutive Secretion)
- aka…
- What is there to aid in continuous secretion?
- Transport Vesicles from…
- Examples: (2)
- continuous secretion
- Specific protein sequence tags target proteins
- the TGN move directly to the plasma membrane and continuously fuse to the plasma membrane (unregulated secretion)
- Ex: mucus secretion, many plasma membrane proteins
(Protein Sorting/Secretory Pathways)
(2. Regulated Secretion)
- Other specific…
- Transport vesicles from…
- Undergo…
- Secretion is then…
- Examples: (3)
- protein sequence tags target proteins for regulated secretion
- the TGN mature into secretory vesicles that are retained in the cytoplasm
- condensation, concentration of materials in the vesicle
- regulated by some signaling process
- Ex: Neurotransmitter release, zymogen granules, hormones (insulin…)
(Membrane Polarity)
Onset of polarity begins at
the ER and continues through the Golgi, to the plasma membrane and other organelles.
(Membrane Polarity)
What occurs in the RER & Golgi Lumen?
Protein Glycosylation & Sulfation
(Membrane Polarity)
After the TGN, the intracisternal lumen becomes
the lumen of transport and secretory vesicles
(Membrane Polarity)
- Upon fusion to…
- Thus, there is…
- the plasma membrane, the luminal side becomes the extracellular side
- a much higher glycosylation & tyrosine sulfation on the extracellular surface of the plasma membrane, than the intracellular surface
Specific Example of a protein destined for the regulated secretion pathway =
GLUT4
GLUT4 is a
glucose transporter
GLUT4 is associated with
the facilitated diffusion of glucose into cells (adipose & muscle)
GLUT4 has important role in
regulating glucose balance
Implications in diabetes (type II), obesity…
GLUT4 is one of
13 sugar transporters
GLUT4 contains
12-transmembrane domains
GLUT4 is seneitive to
insulin
GLUT4’s ability to…
transport glucose is not majorly influenced by insulin
- The amount of GLUT4 in the…
- Thus…
- cell membrane is responsive to insulin
- in response to insulin the rate of glucose uptake is increased
(GLUT4 Example)
- The amino terminus is on which side?
- What signal sequence initially sent it to the RER and docked with the SRP receptor?
- Intracellular side
- ???
GLUT4 is synthesized in
the RER
GLUT4 has multiple…
with combinations of…
- transmembrane domains
- start and stop transfer sequences
- GLUT4 also has…
- The N-terminus of GLUT4 contains sequences that target…
- The C-terminus of GLUT4 contain sequences that target…
- Multiple internal sequences for protein targeting
- the transporter to remain inside the cell in a group of intracellular vesicles (regulated secretion)
- GLUT4 to a pool of intracellular vesicles that are insulin sensitive (a specific subset of regulated secretory vesicles)
GLUT4 has a single…
Typical…
Glycosylation seems to be…
- site of N-glycosylation
- N-glycosylation via oligosaccharide protein transferase
- important for efficient targeting of GLUT4 towards specific subcellular compartments (but not required)
GLUT4 Trafficking Steps: (4)
- ) GLUT4 is packaged into transport vesicles at the TGN
- ) The vesicles containing GLUT4 are then directed to the cell membrane where they are tethered and docked to the membrane (but not fused)
- ) Insulin signaling causes them to fuse to the membrane
- ) The GLUT4 transporters can be recycled to endosomes, TGN and re-tethered….
