1-37 Cytoskeleton I (Microfilaments) Flashcards
What are the three types of ubiquitous filaments in eukaryotic cells?
- Intermediate filaments provide strength; not usu. dynamic
- Microtubules organize cytoplasmic organelles, move chromosomes, and are dynamic
- Actin filaments are dynamic and important for cell shape determination
What are the structure and polarity of actin filaments?
Structure: actin monomers in double-stranded, 5-7nm diameter, helical filaments of variable length.
The uniform orientation of monomers results in structural polarity of filaments. One end of each filament is designated plus (barbed), while the other end is minus (pointed). Structural polarity can be determined by growth rate (plus end grows quickly) and by electron microscopic appearance after binding myosin S1 fragments (plus end barbed).
How do actin filaments assemble and organize?
Hydrolysis of ATP→ADP on actin filament subunits results in different assembly equilibria at the plus and minus ends and leads to treadmilling.
In vivo actin assembly is regulated largely by two classes of actin monomer binding proteins, thymosins and profilins, which also affect the length, number, organization, and in vivo function of microfilaments. They can generate force (myosin), regulate motility (troponin), crosslink (filamin), bundle (α actinin and fimbrin), cap filaments (α/β capping protein) and anchor filaments to other structures such as the plasma membrane (talin).
How do actin filaments interact with, support, and form extensions of the plasma membrane?
Microfilaments may be crosslinked to each other to form meshworks or bundles (both parallel and non-parallel), providing strength and shape to the cell membrane (A-D), or making a scaffold for some forms of motility.
- *A) Microvilli:** e.g., in absorptive intestinal cells
- *B) Stress fibers:** antiparallel, contractile bundles of actin filaments that terminate on the plasma membrane at focal contacts
- *C) Lamellipodia:** extensions of the cell membrane in a migrating cell
- *D) Contractile ring** of a dividing cell
Plasma membrane + underlying mesh of crosslinked actin filaments = cell cortex
How is the ARP complex involved in actin filament-plasma membrane interactions?
The ARP (Actin Related Protein) complex mediates assembly of a branched array of actin filaments at the leading edge of an extending cell membrane (lamellipodia), as occurs when fibroblasts are migrating through the extracellular matrix in connective tissue.
- Assembly near the membrane is essential for pushing out the thin lamellar edge of the cell
- Capping protein limits the length of growing filaments
- Disassembly away from the edge regenerates actin monomers for new rounds of assembly
- Only works if the ARP complex is activated at the receptor and if the actin is branched properly
In non-muscle cells, how do the structure and function of myosin I and myosin II compare?
- Myosin I does not form filaments. Individual myosin molecules bind to membranes and walk along actin filaments toward their plus ends
- Myosin II assembles into bi-polar filaments, with heads that face two directions. To create movement, they interact with two overlapping sets of actin filaments polarized in opposite directions.
How do myosin motility and actin assembly/disassembly coordinate to power cell motility?
- Actin assembly extends the lamellipodium
- Attachment to the substrate and contraction of microfilament bundles pulls the center of the cell forward
- Detachment of the back end of the cell and further contraction brings the back end of the cell forward
What are sarcomeres?
Sarcomeres, which make up myofibrils, are anchored bundles of interdigitating actin and myosin filaments.
- The plus ends of actin filaments attach to Z discs
- The minus ends extend away from Z discs but do not overlap in the center of sarcomeres
- Bipolar myosin filaments (thick filaments) are held in the middle of each sarcomere (by titin), extend from the Z-disc to the thick myosin filaments, and act to center the myosin filaments in the sarcomere
What are myofibrils?
Myofibrils are contractile protein bundles that make up skeletal muscle fibers and that are themselves divided into sarcomeres.
- Myofibrils are bound to each other by desmin intermediate filaments
- The entire array is anchored to the plasma membrane by many proteins, one of which is dystrophin
- Each myofibril is separated from its neighbor by specialized endoplasmic reticulum membrane compartments, called the sarcoplasmic reticulum, which functions to store and release Ca2+
By dividing the entire muscle cell into myofibrils, which are themselves subdivided, the muscle can function as a collection of small contractile machines.
How does calcium regulate actin-myosin interactions in non-muscle cells?
Ca2+ concentration changes indirectly activtes phosphorylating protein kinases, causing phosphorylation of light chains.
This mechanism is slow and, therefore, ill-suited to the regulation of skeletal muscle contraction. This regulation is said to be myosin-based or thick filament-based regulation.
What is the mechanism of muscle contraction?
- Rigor: The myosin head is tightly bound to the actin filament and nucleotide-free
- Release: ATP binding to the myosin head lowers myosin’s affinity for actin → release
- Cocked: ATP hydrolysis causes a 5nm translocation of the head, which cocks it in preparation for the power stroke. This configuration has weak affinity for the actin filament
- Force-generating: Inorganic phosphate dissociates, increasing the affinity of the myosin head for the actin filament and activating the power stroke
- Attached: Translocation of the myosin head back to its original configuration stimulates ADP dissociation
How does the troponin-tropomyosin complex participate in the regulation of skeletal muscle contractions?
AKA actin-/thin filament-based regulation:
- Troponin T binds to tropomyosin, thereby positioning the complex on the actin filament
- Troponin I binds to actin filaments and affects tropomyosin positioning on the actin filament
- Troponin C binds Ca2+ ions
Depending on the presence/absence of Ca2+, tropomyosin can associate w/ actin filaments in a configuration that allows myosin binding or blocks it, respectively
How does calcium regulate actin-myosin interactions in muscle contractions?
By Ca2+ influx into myofibrils:
- An action potential is transmitted: nerve cell → down the plasma membrane → transverse tubules → sarcoplasmic reticulum
- Action potential sensed by a voltage gated Ca2+ channel imbedded in the transverse tubule
- Ca2+ release channels in the SR membrane open into the cytosol surrounding the myofibrils
- Nerve impulse stops
- CaATPase pumps in the SR membrane pump the Ca2+ back out of the cytosol
- Myosin is prevented from interacting with actin
- Contraction stops, and the muscle relaxes
