Cells and the Cytoskeleton Flashcards
1
Q
Transport of Proteins to the Nucleus
A
- proteins are not synthesised in the nucleus
- to act in the nucleus proteins have to be imported from the cytosol
- to allow this to happen there are nuclear pores in the nuclear membrane that are policed by proteins
2
Q
Nuclear Pores
A
- 9nm in diameter
- made up of more than 50 different proteins called nucleoporins
- freely permeable to molecules <5000MW (e.g. ATP, cofactors, small metabolites)
- molecules ~17000MW can equilibrate slowly through the pores (e.g. small proteins)
- molecules >60000MW require active transport (e.g. larger proteins)
3
Q
Nuclear Targeting Signals
A
- usually a stretch of basic amino acids
- the exact sequence and location in the protein is not usually important
- but the signal will be on the surface of the protein
- a single point mutation can totally abolish the function of the sorting signal
4
Q
Nuclear Import
A
- protein with nuclear import signal binds to import receptor
- this causes a change in conformation of the receptor which interacts with a nuclear pore complex
- the receptor then spontaneously carries the protein through the pore and into the nucleus
- recycling of the receptor back out of the pore requires energy
- in the nucleus Ran-GTP binds to a different site on the receptor
- GTP is hydrolysed providing the energy required to transport the receptor out through the nuclear pore
- in the cytosol Ran-GDP is released and the receptor is free to bind again
- a GEF in the nucleus replaces GDP with GTP to reset the Ran-GTP
5
Q
Nuclear Export
A
- Ran-GTP and a protein with a nuclear export signal both bind to an export receptor in the nucleus
- Ran-GTP hydrolyses GTP to release the energy required for transport to the cytosol
- in the cytosol, Ran-GDP and the protein are released
- the receptor is spontaneously transported back into the nucleus through the nuclear pore
6
Q
What are the four locations in a mitochondrion that a protein can be targeted to?
A
- matrix
- inner mitochondrial membrane
- intermembrane space
- outer mitochondrial membrane
7
Q
Mitochondria
Intermembrane Space
A
- similar to the cytoplasm
- contains enzymes that use ATP
8
Q
Mitochondria
Inner Membrane
A
- highly folded
- 5x the surface area of the outer membrane
- contains respiratory chain molecules
- contains ATP synthetase and transport proteins
9
Q
Mitochondria
Matrix
A
- mitochondrial genome
- proteins synthesis machinery
- variety of enzymes
10
Q
Mitochondrial Protein Targeting
A
- nuclear encoded mitochondrial proteins have a signal sequence at the N terminus to target them to the mitochondria
- there is no specific primary amino acid sequence
- the sequence is enriched in basic, hydrophobic and polar amino acids
11
Q
Protein Transport to the Mitochondrial Matrix
A
- the outer mitochondrial membrane contains TOM complexes and the inner mitochondrial membrane contains TIM complexes
- a TIM and a TOM complex can align with each other to form a channel from the cytosol to the matrix
- they interact with each other to form a contact site at which proteins can cross both membranes
- once in the matrix the targeting signal is cleaved off by an enzyme
12
Q
Chloroplasts
A
- just one member of a family of organelles called plastids
- 3 membranes
- bigger than mitochondria
- have a bigger genome than mitochondria
13
Q
What are the 6 locations in a chloroplast that proteins can be targeted to?
A
- thylakoid lumen
- thylakoid membrane
- stroma
- inner membrane
- intermembrane space
- outer membrane
14
Q
Protein Transport to the Thylakoid Lumen
A
- this requires to signals on the protein
- the chloroplast transit peptide targets the protein to the stroma
- to enter the stroma the protein passes through two protein complexes, TOC in the outer membrane and TIC in the inner membrane
- TOC and TIC align creating a channel for the protein to ass through, this process requires energy
- once in the stroma this signal is cleaved off
- from the stroma the protein is then targeted to the thylakoid space by the thylakoid signal sequence
- this process is similar to entry into the RER
15
Q
Complex Plastids in Protists
A
- complex plastids in protists formed by a secondary endosymbiosis event when a photosynthetic eukaryote was engulfed by another eukaryote
- as a result their plastids have three or more membranes
- proteins targeted to complex plastids are first targeted to the ER and then (possibly via vesicles) to the plastid
16
Q
Peroxisomes
A
- contains enzymes for fatty acid metabolism
- single membrane
- no DNA
- like mitochondria they import proteins post-translationally
- unlike mitochondria and chloroplasts they can import folded proteins
- in mammals peroxisomes and mitochondria cooperate to break down fatty acids
- in plants and yeast, peroxisomes are the only place in the cell where fatty acids are degraded
17
Q
Protein Targeting to Peroxisomes
A
- peroxisomal proteins are targeted in two ways
1) soluble proteins use a C terminal SKL amino acid signal and variants e.g. SRL. this is a transplantable signal
2) membrane proteins are though to be targeted to the peroxisome from the ER but this is under debate, it is possible that the peroxisome is a domain of the secretory pathway that has evolved the ability to import proteins from the cytosol
18
Q
What is the purpose of the cytoskeleton?
A
-maintenance/modification of cell shape and promotion of movement of and within the cell
19
Q
What three filaments make up the cytoskeleton?
A
- actin filaments
- intermediate filaments
- -microtubules
20
Q
Actin Filaments
Characteristics
A
- 2 stranded helical shape
- 5-9nm in diameter
- flexible
- organised into parallel bundles creating gel like properties
- form 2D networks under the surface of the plasma membrane
- there is membrane anchored actin and soluble actin
21
Q
Actin Filaments
Function
A
- determine cell surface shape
- generate specialised cell surfaces e.g. microvilli
- mediate whole cell locomotion
- movement along a surface by ‘creeping’ is controlled by the selective polymerisation and depolymerisation of actin under the surface of the plasma membrane
