Lecture 3 - Cellular Aspects of Development – Cytoskeleton; Control of Cell Shape and Intracellular Movement Flashcards
what is the cytoskeleton?
Intracellular networks of protein filaments of several types: Microtubules, Actin microfilaments, Intermediate filaments
what are the cellular aspects of development involve the cytoskeleton?
- Acquisition of polarity
- Control of cell size and shape
- Control of cell division
- Intracellular movement of components
- Cell movement and adhesion
what are microtubules?
Microtubules are composed of the protein tubulin, which exists as a dimer of two closely related subunits, a and b tubulin
Both tubulin subunits bind guanosine triphosphate (GTP) and this is important in regulating microtubule formation
GTP bound to b tubulin (but not a tubulin) can be hydrolysed to GDP
what are polymers of tubulinr?
Microtubules consist of 13 protofilaments, each of which is a polymer of many tubulin dimers
Microtubules are dynamic structures which undergo continual assembly and disassembly
The growth and shrinkage of microtubules is regulated by guanosine triphosphate (GTP).
Both a and b-tubulin bind GTP
Tubulin dimers bound to GTP are added at the ‘plus’ end of the microtubule.
GTP bound to b-tubulin hydrolyses to GDP shortly after addition to the microtubule.
Tubulin dimers containing bound GDP are lost from the minus end of the microtubule
If [tubulin-GTP] is low, the rate of addition at the + end is slow and GTP hydrolysis will remove the GTP cap. Under these conditions tubulin is rapidly lost from the + end, resulting in complete depolymerisation.
what is microtubule elongation or shrinkage?
assembly = elongation
disassembly = shrinkage
what are actin microfilaments?
The globular actin monomer is called G-actin and it polymerizes into F-actin filaments.
The filaments consist of a tightly wound helix ~7 nm in diameter.
Actin binds ATP or ADP.
Hydrolysis of ATP to ADP follows polymerization.
The filaments have a +/- orientation and monomers are added mainly at the + end.
The drug cytochalasin B binds at the + end preventing elongation
what is the Assembly/disassembly is a dynamic process and is regulated by several proteins?
Cofilin binds to filaments and promotes disassociation from the minus end.
Profilin promotes ATP binding to actin and promotes polymerization.
Arp2/3 proteins act as nucleation sites to stimulate assembly of new filaments.
what are intermediate filaments?
Have a diameter of ~10 nm
Composed of various types of proteins (~50 expressed in different types of cells)
The different filaments have a similar basic structure
how do actin filaments assemble?
into bundles and networks held by different types of cross-linking proteins
what is the filament assembly?
Involves multimerization
No +/- ends
More stable than microtubules and microfilaments
what is the function of intermediate filaments associate with?
other cytoskeletal elements, the plasma membrane and organelles. They help to increase mechanical strength and anchor components.
how do you visualise cytoskeletal components?
Cytoskeletal proteins can be visualized when they bind a fluorescently labelled ‘tag’ or specific antibody introduced into cells. The cells are examined by fluorescence microscopy.
what roles do cytoskeletal components have in development?
Look at:
Control of cell shape
Control of intracellular transport: polarity
how do you control the cell shape?
Both microtubules and actin filaments are very important in controlling cell shape.
Actin bundles and networks underlie and support the plasma membrane
E.g. 1 Human erythrocytes lack microtubules and intermediate filaments, so their shape is controlled by the actin network.
The actin network is connected by spectrin cross-linking protein and anchored to the plasma membrane by ankyrin.
E.g. 2 Bundles of actin filaments support protrusions such as microvilli of intestinal epithelial cells
E.g. 3 The trichome leaf hairs of Arabidopsis are branched single cells.
Plants treated with drugs that disrupt microtubule (left) and actin (right) polymerization produce abnormally shaped trichomes.
how do you control intracellular movement?
Movement of cellular components is vital for some developmental processes.
Both microfilaments and microtubules are involved in controlling intracellular movement.
where do the MTOCs assemble?
In many cells microtubules assemble from microtubule organising centres (MTOC’s).
In animal cells the major MTOC is the centrosome, which often contains 2 centrioles. The microtubules radiate outward from the centrosome.
Plant cells do not have centrosome MTOC’s.
what are microtubules involved in?
-intracellular movement e.g. of chromosomes at mitosis
Microtubules are also involved in intracellular movement of vesicles and organelles
Movement along microtubules involves motor proteins called kinesins and dyneins
These proteins use the energy of ATP hydrolysis to move components
what is the bicoid mRNA?
E.g. Bicoid mRNA is synthesised in the nurse cells of the maternal ovaries and transferred to the oocyte where it becomes localised at the anterior end. Other mRNAs also show polar distribution.
This movement is prevented by drugs which inhibit tubulin polymerization.
Localization requires mRNAs binding to microtubules via linker proteins. The mRNA-protein complexes move along the microtubules bound to kinesin motor protein
what does kinesin move? compared to dynein
‘cargo’ towards the + end of microtubules
Whereas dynein moves cargo towards the - end
what is the arp2/3 mutant?
Several mutants with abnormal trichomes lack Arp2/3 proteins required for normal actin filament production
The actin cytoskeleton is disrupted in mutants defective in Arp2/3 proteins
how do microtubules differ in stability?
Microtubules differ in stability. In some cells the half-life is only a few minutes, allowing the cytoskeleton to be continually remodelled.
Stability is affected by a variety of factors. E.g. low temperature causes depolymerization.
The drugs colchicine and colcemid bind tubulin and prevent polymerization.
Stability is also regulated by microtubule-associated proteins (MAPs).
