Lecture 2. Protein Transport Mechanisms in Cells Flashcards
What is the lipid bilayer?
Lipids contain a hydrophobic head and hydrophyllic tail. Lipids come together to form a bilayer to bury the hydrophobic tail in aqueous environments
What do internal membranes do ?
They create enclosed compartments that segregate different metabolic processes
What is the nucleus surrounded by ?
A double membrane called the nuclear envelope
Where are proteins synthesised ?
On ribosomes
Where can ribosomes be found ?
- Free in the cytosol
2. On the surface of rough endoplasmic reticulum
What do cells need to do constantly to function ?
Deliver proteins to their appropriate organelles within the cell
What do signal sequences do ?
Direct proteins to the correct compartment
What are signal sequences composed of ?
A sequence of amino acids
What is a sorting signal ?
A signal sequence of a protein
What are the three protein transport mechanisms ?
- Transport through nuclear pores
- Transport across membranes
- Transport by vesicles
What is on the nuclear envelope ?
Nuclear pores
What are nuclear pores ?
The exit and entry for all molecules entering and leaving the nucleus
What must larger molecules have to move out through nuclear pores ?
The correct signal sequence
What must proteins in the cytosol have to gain entry to the nucleus through the nuclear pores ?
Nuclear localisation sequence
What way are proteins transported through the nuclear pores ?
In their fully folded form
What are nuclear pores made up of ?
Multiple copies of a set of about 30 proteins called nucleoporins
What projects out from both sides of a nuclear pore ?
Protein fibrils
In the cytosol, what are nuclear localisation sequence containing proteins bound by ?
Nuclear transport receptors
What do nuclear transport receptors do ?
Interact with fibrils and guide itself and the protein to the pore and from there to the nucleus
What happens once the protein is delivered to the nucleus?
The nuclear transport receptor returns to the cytosol via the nuclear pore
What provides the energy required for nuclear transport ?
GTP hydrolysis
What happens once the nuclear transport receptor directs the protein to the nucleus ?
A Ran protein binds to GTP, which then binds to the nuclear transport receptor causing it to release the protein
What happens after the protein is released in to the nucleus ?
The nuclear transport receptor remains bound to Ran-GTP and gets transported back to the cytosol via the nuclear pore.
What happens nuclear transport receptor and its bound Ran GTP once back in the cytosol ?
The Ran GTP is hydrolysed to Ran GDP, causing the nuclear transporter receptor to be released
What do proteins destined for the mitochondria have ?
A signal sequence
In transport across membrane, where is the signal sequence recognised and bound ?
The transport outer membrane complex
In transport across membranes, where does the bound receptor go ?
It diffuses along the membrane to a contact site on the inner membrane (translocase of the inner membrane)
In transport across membrane, where does the protein move to and from ?
Translocates across the outer and inner membrane
In transport across the membrane, what is the role of chaperone proteins ?
Assist in the translocation and refolding once inside the organelle
When is the signal sequence/peptide removed ?
Once the protein reaches its destination
What are mitochondria ?
Organelles that have their own DNA and can make their own proteins
What do precursor proteins destined for mitochondria have ?
A short segment of amino acids, the signal sequence that targets the proteins to this organelle
What does the signal sequence of a precursor protein have an affinity for ?
A receptor on the surface of a mitochondria
How/where does the precursor protein at the contact site of the two mitochondria membranes move?
Snakes in an unfolded state through two sequential protein translocators
What do the chaperone proteins prevent ?
The protein chain backsliding through the translocator tunnel ?
What does the enzyme signal peptidase do ?
Cleaves the signal sequence from the precursor protein
What is the entry point of proteins destined for the golgi, endosomes, lysosomes, cell surface and the endoplasmic reticulum ?
The endoplasmic reticulum
Do proteins re-enter the cytosol once inside the endoplasmic reticulum ?
No, they will be transported by vesicles to other organelles, the plasma membrane or secreted from the cell
What happens when the endoplasmic reticulum sequence is detected by ribosomes if it is in the cytosol ?
It will direct itself to the endoplasmic reticulum and allow protein to be threaded across the endoplasmic reticulum membrane as it is synthesised
What happens to soluble proteins in transport across membranes ?
They will be directed through the translocation channel on the endoplasmic reticulum and released into the endoplasmic reticulum lumen
What allows proteins to be embedded in the endoplasmic reticulum membrane ?
Membrane proteins have hydrophobic stop and start transfer sequences. This gives rise to transmembrane proteins
How is a polyribosome formed ?
As a MRNA molecule is translated into a protein, many ribosomes bind to it
What are free ribosomes ?
Unattached to any membrane
What are membrane bound ribosomes ?
Ribosomes that become riveted to the ER membrane and translate proteins that are translocated into the ER
Where are membrane bound ribosomes found ?
The surface of the ER creating rough endoplasmic reticulum
How do water soluble proteins cross from the cytosol and the ER ?
They completely cross the ER and are released into the lumen
How do transmembrane proteins cross from the cytosol to the ER ?
They partially cross the ER and become embedded in the membrane
How are proteins directed to the ER ?
A signal sequence of small hydrophobic amino acids
How does the signal sequence get to the ER ?
Guided by a signal recognition particle
What does the signal recognition particle bind to ?
The ER signal sequence in the new protein
When is the signal recognition particle released ?
After protein synthesis slows down to allow time for the signal recognition particle binds to the signal recognition particle receptor in the ER membrane
What happens when the signal recognition particle is released ?
The ribosome is passed to a protein translocation channel in the ER membrane
What other things does the signal sequence do ?
To open the translocation channel
How is the protein chain moved through the membrane ?
The signal sequence remains bound to the channel but the the rest of the protein chain is threaded through the membrane as a large loop
What happens to the signal sequence/peptide once it is cleaved from the protein chain ?
It is released from the translocation channel and enters the membrane where it rapidly degrades
How is the inactive channel closed ?
A protein serving as a plug the binds from the ER lumen
What initiates translocation of a transmembrane protein ?
The N-terminal signal sequence
How is the translocation process of a transmembrane protein stopped ?
a sequence of hydrophobic amino acids called a stop transfer sequence
Where is the stop transfer sequence released from ?
Released laterally from the translocation channel
What happens the stop sequence in the lipid bilayer ?
Forms a membrane spanning segment which anchors the protein in the membrane
How does transport from the ER to the golgi occur ?
Budding and fusion of membrane enclosed vesicles
What is the function of transport vesicles ?
Mediate the passage of proteins inwards from the plasma membrane to lysosomes and outwards from the ER to the plasma membrane
What drives vesicle budding ?
The assembly of the protein coat
What drives vesicle budding ?
The assembly of the protein coat
What may the coat be composed of ?
- Clathrin
2. Coat protein complex
Which vesicles usually coated with clathrin proteins
Vesicles originating from the plasma membrane and golgi, destined for endosomes or lysosomes
Which vesicles are usually made from coat protein complex proteins ?
Vesicles shuttling between the ER and golgi
What do adaptins do ?
Mediate contact between receptors and clathrin molecules that act to shape the vesicle forming at the plasma membrane
What are triskelions ?
Individual clathrin molecules that are seen as three legged structures
How are vesicles transported ?
Along cytoskeletal fibres
What is the vesicular transport system responsible for ?
Protein secretion from cells
How are secretory proteins released from the cell ?
Exocytosis
How is endocytosis mediated ?
By plasma membrane vesicles budding into the cell cytosol
What causes low density lipoproteins to receptors to release their cargo ?
The low internal pH of the endosome
What happens to low density lipoprotein receptors once empty ?
Recycled to plasma membrane in vesicles that bud off from the endosome
What does the lysosome contain ?
A hydrolytic enzyme that digests low density lipoproteins
Where are secretory proteins synthesised ?
Ribosomes on the endoplasmic reticulum
