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
22
Q
Nucleation
Definition
A
-formation of a new structure by via self-assembly or self-organisation
23
Q
Nucleation of Actin
A
- an ARP2/3 complex mimics the beginning of an actin filament
- the actin polymerases from this point to form a filament
24
Q
Microtubules
Characteristics
A
-polymers of tubulin
-hollow cylinders
-25nm in diameter
-thickest
-very rigid
usually attached to a microtubule organising centre (MTOC or centrosome)
-nucleation is similar to that of actin
25
Microtubules
| Function
- provide 'highways' through the cell
| - help to direct intracellular transport
26
Centrosome
- centriole pair - made of modified microtubules
- in a fibrous centrosome matrix
- gamma tubulin ring complexes are also found in the centrosome matrix, they look like the end of a microtubule and are the nucleation point for microtubules
- new microtubules grow out of these gamma ring complexes, out of the centrosome and into the cell
27
Intermediate Filaments
| Charatceristics
- polymers of heterogenous protein family
- cylinders of 10nm
- reasonably flexible
- organised into rope like fibres
28
Intermediate Filaments
| Function
- give mechanical strength to the cell
- resist shear stress
- some extend across the entire cytosol, others form specialised structures e.g the nuclear lamina
29
Examples of Intermediate Filaments
- a network of nuclear lamins (intermediate filaments) form the nuclear lamina under the surface of the nuclear envelope, chromatin can attach to it and it stabilises the nuclear envelope
- keratins - found in epithelial cells and derivatives (nails and hair)
30
Nucleation of Intermediate Filaments
- the keratin cycle
| - reversable
31
Cytoskeleton
| Accessory Proteins
control assembly and attachment in certain locations
32
Cytoskeleton
| Motor Proteins
move organelles along filaments and move the filaments themselves
33
Mysosin
- tightly woven coiled domain composed of two heavy chains
| - two myosin heads at the N terminus
34
Filaments of the Cytoskeleton can be Associated with other Proteins
- cross linking of fibres to form a rigid network
- molecular motors
- modifying filament dynamics
35
Actin Based Molecular Motors
- myosin chains overlap with actin filaments
- each myosin head can bind ATP and hydrolyse it to release energy that they use to slide over an actin filament
- muscular contraction is simply a change in protein conformation requiring energy
36
Microtubule Based Molecular Motors
-kinesin and dynein form links between microtubules and microtubules or proteins
37
Cell Adhesion
- cell to cell adhesion needs to involve membrane spanning proteins
- membrane proteins from one cell bind to membrane proteins in another cell
- many of these proteins have to bind in order for two cells to adhere
- but for this to work, the membrane proteins also have to be attached to the cytoskeleton
- there is no strength in the bond between the transmembrane domain of a membrane protein and the plasma membrane as the lipid bilayer is mostly liquid
- only the cytoskeleton of cells can mediate adhesion
38
Which filaments mediate cell adhesion?
-actin filaments and intermediate filaments can both mediate cell to cell adhesion and cell to extracellular matrix adhesion
39
Adhesion Belts
- actin filament mediated
- cadherin dimers connect the membrane proteins in the membranes of two adjacent cells
- they are connected to anchor proteins / adapters
- actin filaments are also connected to the anchor proteins
- adhesion belts can shape parts of tissues by changing the shape of the surface of the cell plasma membrane
40
Desmosomes
- intermediate filament mediated
- button like zone of intercellular contact
- cadherin family proteins connect cytoplasmic plaques (made of intracellular anchor proteins) in adjacent cells
- intermediate filaments (e.g. keratin) are anchored to the cytoplasmic plaque
41
Tight Junction
- the major protein types are claudins and occluding
- extracellular domains of transmembrane proteins in the plasma membranes of adjacent cells bind together
- this creates a virtually impermeable barrier to fluid between the two cells
- this prevents the diffusion of membrane spanning proteins pasts the tight junction allowing cell polarisation to occcur
- they also prevent the movement of molecules between cells
42
Gap Junctions
- found in animal cells
- characterised by a high concentration of protein channels
- a series of connexion proteins line up to form a channel in the plasma membrane called a connexon
- connexons in the plasma membranes of adjacent cells can line up to form an intercellular channel
- small peptide, amino acids, sugars, ions, nucleotides and ATP can be exchanged through gap junctions
43
Plasmodesmata
- found in plants
- carry out the same function as gap junctions in animal cells
- channels between plant cells
- all plasma membranes and ER are connected through plasmodesmata
- but they have a completely different structure than gap junctions
44
What materials make up the extracellular matrix?
- collagens
- fibronectins
- elastins
- laminins
45
Cell to Extracellular Matrix Adhesion
- integrins form the linkage between the extracellular matrix and the cytoskeleton
- they bind to elastins or collagen in the extracellular matrix
- this usually occurs in tissues where cells are imbedded in a very large matrix without adjacent cells nearby
46
Extracellular Matrix
| Collagen
- three intertwined chains form a collagen molecule
| - collagen biosynthesis occurs in the secretory pathway
47
Extracellular Matrix
| Elastins
- cross linked to each other
| - are able to relax and stretch permitting recoil
48
Cell Differentiation
| How do cells know when to divide?
- establishment of cell polarity
- division in different planes
- cell to cell contacts and interactions
49
Cell Differentiation
| How do cells know which portion of their genome to use?
- tissue specific gene expression
| - signalling from one part of the body to another
50
Definition of Cell Polarity and Examples of Polarised Cells
- one side of the cell is not the same as the other side of the cell
- e.g. spore formation occurs by asymmetric cell division
- flagella are only present on one side of a cell
51
Protists
- among the most complex eukaryotic cells
- very versatile with respect to motility
- free living protists often carry out all the specialised functions of multicellular organisms and have large genomes
- sometimes have primitive 'legs' and can 'walk' on surfaces
- some have complex skeletons often ornamented with barbs or even branched projections
- often capable of directed predation
52
Basic Principle of Polarity
- separation of membrane zones by molecular fences
| - functionally different cell sub-compartments
53
Syncytium
- cells can fuse together forming a syncytium, a multinucleate giant cell
- this can happen during normal development or can be induced by pathogens and symbionts
54
Examples of Polarised Cells
- nerve cells
- sperm cells
- goblet cells
- mammory cells
- photoreceptor cells
