Motility, chemotaxis and intermediate filaments Flashcards
What is chemotaxis?
The ability to sense and move towards or away from a signal
Describe the steps of locomotion
1- Extension of Lamellipodia from edge of leading cell via actin polymerisation pushing membrane forward
2- New focal adhesion (feet) are made, via capping actin filament bundles.
3- Bulk of cytoplasm pushed forward via contracting Actin-Myosin II bundles (stress fibres) by translocation
4- Trailing edge of cell detaches from EM, during this Integrins are internalised
Describe the steps of chemotaxis
1- During chemotaxis the intracellular distribution of each class of GTPase is controlled so that a gradient is formed within the cell.
2- At the leading (growing) edge Cdc42 activation promotes formin and Arp2/Arp3 dependent actin assembly to promote filopodia and lamellipodia growth.
3- This in turn stimulates Rac GTPase which further promotes branched actin network assembly behind the leading edge.
4- Rac activation leads to Rho GTPase activation at the lagging edge of the cell which promotes stress fiber formation and myosin II activation which powers forward movement of the bulk of the cell contents.
5- This systems is highly dynamic as the cell can rapidly change direction in response to changes in concentration and direction of chemotactic signals.
How is actin assembly regulated by the Rho family GTPase?
Dom Rho - Stress fibres
Dom Rac - Lamellipodia
Dom Cdc-42 - filapodia
What is chemotaxis important for?
Wound healing (fibroblast migration) Inflammatory response (neutrophil migration)
What makes up cilia/flagella?
9 x MT doublet
What is the structure of the axoneme of cilia and flagella?
Axonemal Dyneins come in multiple forms that contain either one, two or three non-identical heavy chains. Each heavy chain has a globular motor domain, a “stalk” that binds to the microtubule and an extended tail (or “stem”) that attaches to a neighbouring microtubule of the same axoneme. Each Dynein molecule thus forms a cross-bridge between two adjacent microtubules of the axoneme.
What governs the power stroke of cilia and flagella?
The motor domain undergoes a conformational change that causes the microtubule-binding stalk to pivot relative to the cargo-binding tail with the result that one microtubule slides relative to the other.
This sliding produces the bending movement needed for cilia to beat and propel the cell.
Groups of dynein molecules responsible for movement in opposite directions are activated and inactivated in a coordinated fashion so that the cilia or flagella can bend and flex co-ordinately.
How does Nexin govern the power strokes
The doublets are cross-linked by Nexin and make contacts to the central pair of singlet MTs.
These cross-links transmit a sliding force between the tubules.
If an isolated axoneme is mildly proteolyzed (which removes Nexin) and ATP added, the tubules slide on each other, rather than bending.
Remember dynein walks from + to - along a microtubule (I.e. towards the basal body).
Difference between MF, MT and IF
MF and MT generate force
Intermediate filaments resist force.
Describe the structure of IF
It is made up of a head at N term, Rod and Tail at C term.
What are Desmin and Synemin’s role in Type III intermediate filaments
In smooth muscle, Desmin cross-links at dense bodies to provide a resistive force to stretching.
Desmin also prevents overstretch of the sarcomere in skeletal muscle and works in conjunction with two other intermediate filament components, Synemin (circling the Z line) and Skelemin (circling the M line) to maintain sarcomere organisation and integrity.
The thick filament (Myosin II motor bundle) and thin filament (F-actin) do the work of muscle contraction.
What makes up type IV filaments - neurofilaments
Neurofilaments consist of obligate heterodimers consisting of NF-L (light), NF-M (medium) and NF-H (heavy) subunits.
These are required for structural support in axons and glial cells and are frequently bound to, and transported by, microtubules.
What makes up type V IF - The nuclear lamins
The nuclear lamina is located on the inside of the nuclear envelope and provides structural support for it. Nuclear lamins (A, B, C) form dimers in isolation. These then associate to form a meshwork. A and C lamin are splice variants transcribed from the same gene, differing at the C-terminus. The C-terminus of B lamin is covalently attached to the membrane via polyisoprenyloid lipids.
