Cytoskeleton Flashcards
What end of F-actin does G-actin polymerise most quickly?
the plus (+) end
What causes nucleation?
Random collisions, which explains the high rate of polymerisation when concentrations of actin are high
Actin filaments (F-actin) are polymers of:
globular protein actin (G-actin) that contains a bound nucleotide (ATP or ADP).
Actin monomers have ATP attached. What is this ATP used for?
NOT required for polymerisation
the bound ATP influences the stability of the filament ends.
What are the steps in cell movement?
Nucleating protein promotes actin polymerization.
Integrins adhere to surface when a favorable environment has been found. This anchors cell
Actin filament severing proteins break some microfilaments and this changes the cytoplasm from a thick, gel to more aqueous in nature. This allows the cytoplasm to more easily flow towards its leading edge.
Motor proteins (myosins) help to slide microfilaments in appropriate direction
What are the names of the structures located at the leading edges of moving cells?
Lamellipodia - Thin sheet-like structure w/ dense network of f-actin in a single orientation w/ the + end at the plasma membrane
Filopodia - Exploratory fibers, long and thin w/ paralel bundles of 10-20 F-actin microfilaments
Cell pushes out a leading edge. This edge adheres to the surface and forms an anchor. Rest of cell uses these anchoring points to pull itself forward.
True or false; Polymerization and depolymerization can occur at both ends of the F-actin microfilament?
True; While ATP G-actin is being added at the + end (ATP cap), the minus end has hydrolyzed ATP to ADP and this destabizes microfilament
Thus, microfilament is undergoing depolymerization at the - end while undergoing polymerization at the + end
Individual G-actin molecules move down the length of the filament in a process called ‘treadmilling’
Add one to + end and subtract one from - end
G-actin and F-actin ar are at equilibrium
Changes in local conditions can push equlibrium in either direction thus dynamic instability is the rule for microfilaments as well as microtubules
Describe the process of microfilament polymerization.
G-actin can be added at the + end or - end of the microfilament
Rate of growth is faster at the + end
Depolymerizes at both ends
Rate of depolymerization is fastest at the - end
GATP-Actin rate of addition at the + end ie enhanced when F-actin still has ATP bound to individual G-actin subunits
ATP cap forms at plus end of rapidly elongating microfilament
What is the mechanism that drives ciliary /flagellar movement?
The mechanims exist within the 9 + 2 structure
Movement is dependent upon ATP concentration
Cilary dynein is responsible for movement
Dynein arms on A subtubule have a head region where ATP hydrolysis takes place at the cross bridge that links to the B subtubule
cAMP and Ca2+ both regulate beating frequency (When calcium is high intracellularly, via voltage gates channels, motion slows)
Ca2+ - Calmodulin acts on cilia to reverse direction while the rate of forward movement is regulated by cAMP
Describe the structure of cilia and flagella.
The core (axoneme) has a 9 + 2 arrangement of 9 outer doublet rings and 2 inner singlet rings.
The outer doublet ring has 1 complete (A) MT protofilament and 1 incomplete (B) MT filament made up of 9 protofilaments
Inner singlets are made up of two separate MTs made up of 13 protofilaments
Large number of Microtubule Associated Proteins associated with the axoneme
Dynein side arms on the complete A ring of outer doublet
Protein radial spikes connect central sheath surrounding the inner singlets w/ outer doublets
Nexin bridge connects the doublets to one another
Basal body or the part of the cilia/flagella that continues into the cell, has the same structure as the centriole w/ nine triplet rings. It can regenerate cilia/flagella if they are sheared off at the membrane
A and B tubules of basal body continue up to form the A and B tubules of the doublet in cilia/flagella
Describe cilia and flagella.
Cilia are usually very numerous, short and beat in a coordinated fashion. Movement is a power-stroke movement.
Flagella (eucaryotic) are longer and usually 1-2 per cell. Move in multiple waves.
What is the other non-cytoskeletal role of microtubules?
Microtubules make up cilia and flagella and help cells move by the use of a special dynein motor protein.
What are ‘motor proteins’?
Motor proteins are accessory proteins that carry intracellular cargo along the microtubule ‘tracks’.
Two types
Kinesin
Dynein
Both hydrolyze ATP to provide the energy for movement
