Lecture 1 - Cytoskeleton Flashcards
What is the cytoskeleton?
The cytoskeleton is a highly ordered, dynamic network of filaments that extend throughout the cell and collectively responsible for cell shape, motility (movement) of the cell as a whole and positioning of internal structures
The cytoskeleton is very dynamic in nature, with its filaments constantly forming and breaking down.
The cytoskeleton is a dynamic structure made up of proteins that self assemble into long polymers of repeating subunits.
What processes is the cytoskeleton involved in?
Cell morphology Cell migration Vesicle transport Cell division cytokinesis chromosome separation
What are microfilaments?
- Polymers of actin
- carry out cellular movements– and also, play a critical role in muscle contraction and cytokinesis (cell division). Actin cytoskeleton plays important in final step of cytokinesis
What are microtubules?
- Polymers of tubulin
- act as a scaffold to determine cell shape, and provide a set of “tracks” for cell organelles and vesicles to move along. - also form spindle fibers for separating chromosomes during mitosis.
What are intermediate filaments?
- Polymers of various intermediate filament proteins e.g. keratins
- Intermediate filaments provide tensile strength to the cell.
How does eukaryotic cell movement take place?
Dynamic actin cytoskeleton (microfilaments) drives eukaryotic cell movement
Eukaryotic cell migration involves dramatic changes in cell shape, driven by the cytoskeleton - particularly the actin cytoskeleton.
As the cell moves the leading edge forms ruffles, in between processes called lamellipodia and filopodia .
Actin bundles extend from an actin arc into the leading edge while longer fibers (called stress fibers) extend from the arc in the opposite direction into the cell tail.
What are lamellipodia?
Broad membrane extension that moves forward. Typical of rapidly migrating cells
What are filopodia?
Finer cytoplasmic extensions, typical of slower moving cells
What are focal adhesions?
Structures that form mechanical links between intracellular actin filaments and the extracellular substrate
What are ruffles?
Assemblies that do not form tight adhesions with the substrate
What do microfilaments consist of?
globular G-actin monomers (single actin subunit) assembled into filamentous polymers (F-actin, actin filament consisting of multiple actin subunits).
F-actin adopts a tight helical structure with 14 actin subunits being required to make a complete turn of the helix.
Describe the structure of G actin
•globular protein, 2 lobes separated by a large cleft – each lobe consists of 2 domains (4 domains numbered on this slide as I – IV).
•hinge between domains I and III allows the two lobes to move relative to each other forming a nucleotide binding cleft
•nucleotide binding is required for G-actin stability (G-actin is unfolded in the absence of nucleotide)
•structure of ATP- and ADP- bound actin is identical everywhere EXCEPT in subdomain 2 (this is important for actin function – in the process of treadmilling).
•actin monomers have polarity – a plus and a minus end –also called the barbed and pointed end –this polarity is important for actin structure and function.
G-actin monomers have the ability to self assemble into F-actin. This starts with the formation of an actin trimer that serves as a nucleus for filament extension.
How do actin filaments assemble?
Nucleation – 3 monomers form a trimer
Elongation of filament by monomers adding at both ends (in a polarised manner)
Only occurs when the concentration of G-actin monomers is high enough – called the Critical concentration (Cc) for assembly (disassembly occurs if a filament is in a solution containing a concentration of G-actin below this Cc).
What is treadmilling?
- Monomers add to (+) end, dissociate from (-) end at same rate
- Filament length remains constant but moves across surface
