Cytoskeleton Flashcards
What is pinocytosis?
Ingestion of liquids by budding of small vesicles
What is phagocytosis?
Ingestion of whole cells and large insoluble particles
Outline endocytosis
- Coates vesicles formed from coated pits
- Vesicles are uncoated
- Vesicles fuse to endosomes
How are secondary lysosomes formed?
Joining of endosomes and primary lysosomes from phagocytosis of bacteria
Where would you find a targeting sequence?
In a protein destined for a certain organelle
What are the functions of the cytoskeleton?
- Integration of compartments (membrane traffic) and response to cell signals
- Organelle distribution
- Cell motility and division
What are the main features of polymers in the cytoplasm?
- Dynamic
- Rapid assembly and dissassembly
- May be polar
What is the protein concentration in cytoplasm?
~20%
What are the main properties of cytoplasm?
- non-Newtonian- can behave like a solid
- non-uniform- microdomains exist
- resists sudden impacts, melts under slow persistent shear
What diameters are microtubules, microfilaments and intermediate filaments?
Microtubules- 25nm
Microfilaments- 7nm
Intermediate filaments- 10nm
How easily deformed and ruptured are individual actin filaments, microtubules and vimentin networks (intermediate filaments)
Actin filaments- easily d and r
Microtubules- rigid- easily r not d
Vimentin- easily d not r
What reagents can be used to study the cytoskeleton?
- Antibodies- markers for proteins
* Antibiotics- target elements of cytoskeleton
Which antibiotics target microtubules?
Colchicine and taxol
Which antibiotics target F-actin?
Phalloidin and cytochalasins
What instrumentation can be used to study the cytoskeleton?
Low-light digital videomicroscopy
What genetic manipulations can be used to study the cytoskeleton?
- Gene tagging (GFP)
* Modification of gene expression
What are the functions of microtubules?
• Cell shape and polarity • Cell organisation and positioning of organelles • Cell division • Cell motility: -Intracellular transport -Cilia and flagella -Sensory systems
Outline the composition of microtubules
- Tubulin- alpha beta heterodimer
- Assemble head to tail- protofilaments
- 13 protofilaments make up the wall of a microtubule
- Exist in dynamic equilibrium- added to one end, removed from other
Where do microtubules arise and how do they assemble?
- Arise from centrosome- microtubule organising centre (MTOC)
- Assemble with +be end (beta) distal to centrosome
What is a centrosome?
Pair of centrioles enclosed in shell of pericentriolar material (contains lambda tubulin)
What do low temperatures do to microtubules?
Reversibly disassembles then
What effect does cochicine have on a cell?
Depolymerises cytoplasmic microtubules, therefore causes organelle dispersal
(As microtubules determine the 3D organisation of organelles)
What are microfilaments made up of?
Dynamic polymer of G-actin monomers
What effect does cytochalasin have on actin filaments?
Depolymerises them
What effect does phalloidin have on actin filaments?
Stabilises them
How abundant is actin in a cell?
Most abundant protein
10-15%
Outline microfilament assembly?
- Assembly at +ve end, disassembly at -ve
* ATP dependant- one ATP for each monomer lost from -ve
What are actin binding proteins?
Regulate actin filament function
Eg. In the brush border actin filaments are cross linked by villin
Why are intermediate filaments stable during interphase?
No cytoplasmic pool of subunits and filaments
What protein composition do intermediate filaments have?
Alpha helix rich
Alpha type fibrous proteins
(Not globular like mf and mt)
Describe intermediate filament function
- Very heterogenous and tissue specific
* Mechanical integrators of cells into tissues
Describe the composition of intermediate filaments
Monomers are an alpha helix rod and two globular heads
4 monomers form a tetramer filament
What are four intermediate filament proteins and where are they used?
- Lamin- nuclear lamina
- Vimentin- cells of mesenchymal origin
- Keratin- epithelial cells and derivatives
- Neurofilaments- neurons
Where are intermediate filaments found in epithelial cells?
- Keratin filaments- structure
- Nuclear lamina
- Dermosomes (join cells together)
- Hermidermosones (to basement membrane)
- Basement membrane
What makes up a cytoskeletal motor?
An ATPase and a linear polymer with intrinsic polarity
Which ATPases pair with which linear polymers to form cytoskeletal motors?
Myosin with F-actin
Dynein and kinesin with microtubules
What are cross-bridge cycles?
Vectorial movement- direction determined by polarity
Step distance ~8nm
