Lecture 12 - Intracellular compartments and Protein Sorting Part II Flashcards
Peroxisomes
Spherical organelles with a fine granular matrix, sometimes with crystals
The ER is in close contact with
Peroxisomes
Peroxisomes only have a
Single membrane, no DNA
Peroxisomes are
Multi-purpose
Ubiquitous, dynamic organelles with high plasticity
Essential for health
Peroxisomes respond to
Environmental stimuli
Function of peroxisomes (3)
Lipid metabolism (produce lipids) Protective function (detoxification of lipids) Central regulatory role
Peroxisomes produce what during lipid metabolism?
Myelin lipids
Insulation for axons in the brains
Dysfunctions of peroxisomes normally cause
Neurological disease
myelin
In fungi, what is produced in the peroxisome?
Penicillin
In fireflies, what do the peroxisomes do?
Light producing reaction in the lantern organ
Peroxisomes react to environmental stimuli by (3)
- A change in number
- A change in enzyme composition
- A change in positioning in the cell
Peroxisomes can import
Completely folded, oligomeric or co-factor proteins
Co factor Protein
Non-protein chemical compound or metallic ion that is required for an enzyme’s activity
Cofactors can be considered “helper molecules” that assist in biochemical transformations
Oligomer
A macromolecular complex formed by non-covalent bonding of a few macromolecules like proteins or nucleic acids
Peroxisomes can be produced
De novo (dogma breakers)
Examples of how peroxisomes are odd
They can import large proteins that should need a gate without needing a gate (transient pore)
They can form de novo (against biochemical dogma)
PTS
Peroxisome Targeting Signal
Amino acid sequence that targets a protein to the peroxisome
PTS1 is found at the
C terminus of the protein (3 a.a)
PTS2 is found at the
N terminus of the protein (9 a.a.)
PTS1 and PTS2 are recognised by
Specific receptors
Pex 5 and Pex 7 respectively (soluble cytosolic proteins)
Pex 5 and Pex 7 are involved in
Peroxisome biogenesis
Catalase is a key enzyme
In peroxisomes
Degrades H202 produced by Oxidases
Oxidases produce
H202 in peroxisomes
Peroxisomes probably have
A transient pore
Allows them to import large proteins
Not been visualised by electron microscopy
The transient pore model
Pex proteins (Pex5) has bacterial toxin-like properties, can insert into the membrane, interact with lipids
Pex5 forms the pore and releases the PTS1 cargo inside
No ATP required
In the transient pore model, Pex5 is removed by
Pex1, 6, and 26 form a ‘lever arm’
Grab Pex5 when it is ubiquinitated
Pulls it out of the membrane
ATP required
After it is removed from the membrane, Pex5 can either be
Recycled or degraded
If Pex5 is going to be degraded, it is
Polyubiquitinated
Pex5 is degraded by the
26S Proteosome
Ubiquitination requires
ATP
Pex1 and 6 are
ATPases
Pex13, 14 and 17
Aid in tethering and of Pex5 and translocation of the cargo
Example of PBDs
Peroxisome biogenesis disorders
Zellweger syndrome
Cerebro-hepato-renal syndrome
Proteins go into the ER
Co translationally
Co translational transport
While the proteins are made, they go in
‘synthesised into’
Only certain proteins go into the ER (3)
- Extracellular proteins (sent to the Golgi to packaged into vesicles)
- Plasma membrane proteins
- Lysosomal proteins (sent to the Golgi)
and Golgi proteins of course!
SRP
Signal recognition particle
Structure of the SRP
Made out of RNA and 6 proteins
Hydrophobic pocket lined with methionines recognises N terminus signal sequence
The SRP
Has a translational pause domain (like a finger that stops the ribosome translating)
Brings the ribosome to the ER
SRP binds to SRP receptor in the ER membrane
Translation continues and translocaction begins
Once the co translated protein is in the ER
The signal peptide is cleaved and the protein is folded
What does the ER do to the incoming proteins?
Glycosylation (important in extracellular signalling) Disulfide bridging (stabilises proteins) Folding
Quality control in the ER
Checks if proteins are properly folded and will refold or degrade them if they are not
When proteins leave the ER
They are sent to the ER for further processing
Sec61
Protein translocator in the ER
Hetero trimeric complex
Structure of Sec61
Bundled alpha helices with a central pore
Pore is usually closed by a small a-helix ‘plug’
Signal peptide displaces the plug allowing transport of the protein
Two mechanisms of ER transport (4)
- Soluble proteins
- Single pass TM proteins I
- Single pass TM proteins II
- Multi pass TM proteins
ER transport: Soluble proteins
Polypeptide binds to ER signal sequence
Polypeptide is completely transferred into ER lumen
Signal is cleaved
ER transport: Single pass TM proteins I
Polypeptide binds to ER signal sequence
Polypeptide contains stop transfer signal (hydrophobic sequence)
Signal is cleaved
C terminus remains in cytosol
ER transport: Single pass TM proteins II
Polypeptide binds to ER signal sequence
Then:
More positive residues before stop transfer signal = N terminus translocation inhibited
Complete transfer of C terminus part
OR
More positive residues after stop transfer signal = C terminus translocation inhibited
Complete transfer of N terminus part
ER transport: Multi pass TM proteins
(multiple domains have to span the membrane)
Start transfer signals begin
Until a stop transfer signal is encountered
For more than 2 membrane passes, second start transfer signal sequences re initiate translocation
Stop and start transfer signal sequences are
Internal signal sequences
Same sequences, just depends on where they are
Transmembrane transport is used to
Deliver proteins to mitochondria, plastids, peroxisomes and ER
First three occur post translationally, ER is co translational
What do signal sequences do?
Facilitate interactions between polypeptides and receptors/translocators
