Quiz 10

Cytoskeleton: Intermediate Filaments

• Three types of protein filaments in cytoskeleton:
  • Intermediate filaments: robe-like protein fiber.
    • Major framework of cytosol and nuclear lamina.
  • Microtubules: long hollow tube, made of protein tubulins.
    • Major component of mitotic fibers.
    • Provide transport tracks.
  • Actin: helical polymers of protein actin.
    • Mostly underneath the plasma membrane or in muscle.
• Intermediate filaments are composed of \alpha-helix.
• Keratin reinforces epidermal cells.
• Lamina forms a lining supporting the membrane.
• Structures of Intermediate filaments:
  • Two molecules of intermediate filament first twist together to form a coiled-coil dimer.
  • Two ends of the filaments consist of globular domains that connect molecules together end-to-end.
  • Every Intermediate Filament monomer is an intermediate filament that forms an \alpha-helix.
• Functions:
  • Help cell against mechanic stress, increasing the strength of cells.
    • For example, intermediate filaments extend through epithelial cells and prevent these cells from rupture during mechanic stress.
  • Keratins in different cells are connected together by small pores located at the tight junction between cells.
    • This type of intermediate filament is called Keratin filaments.
  • Connect to other proteins and protein fibers such as microtubules with plectin.
• Special type of Intermediate filaments (nuclear lamina) supports the nuclear membrane:
  • Intermediate filaments can be found underneath the nucleus membrane, forming the nuclear lamina, and support the nuclear membrane.
  • During mitosis, Intermediate filaments are disassembled and take the nucleus membrane apart; then assembled after mitosis is done to reform the nucleus membrane.
    • These disassembly and assembly are controlled by phosphorylation.

Cytoskeleton: Microtubules

• \alpha- and \beta-tubulin dimers are the subunits
• Polarity assists movement and transport.
• Functions of Microtubules:
  • They group in bundles, located at the ends of cells (centrosomes) during the interphase of mitosis.
  • During metaphase, microtubules extend out to the center of the cells where they direct chromosomes to two poles of each cell.
  • Microtubules also make up the major mobile component of cilia.
  • During anaphase, microtubules direct chromosomes migrating toward to two poles.
• Microtubules consist of \alpha-tubulin and \beta-tubulin:
  • Each tubulin molecule is made of \alpha and \beta subunits.
  • Pairs of two subunits line up in order to form polarity.
    • \alpha-tubulin end is the minus end and the \beta-tubulin end is the plus end.
  • When tubulins extend, they always grow in the direction of the plus end (\beta-tubulin).
    • This polarity also determines the direction of transport along this track of the tubulin.
  • There are three types of tubulins.
    • Third type of tubulin (\gamma-tubulin) as the base of tubules:
      • \gamma-tubulins at the centrosome serve as the base for microtubules to grow.
      • The first pair of \alpha\beta-tubulins add to the \gamma-tubulin ring (base), and then each pair will add on to the previous one and eventually becomes a long microtubule.
• Assembly and Disassembly of microtubules:
  • When an \alpha\beta-tubulin dimer binds to GTP, the dimer will gain energy and binds to the elongating tube.
  • If the GTP on the tube is hydrolyzed before the next dimer binds to it, the GDP-containing \alpha\beta dimer will fall off from the tube.
  • Assembly (binding) and disassembly (falling off) occur constantly depending on the availability of GTP. (That’s why extension and shortening of microtubules are random.)
• The Assembly and Disassembly can be stabilized by Capping Protein or Chromosomes:
  • If the plus end of the microtubule is permanently bound with some other molecules or cellular structures (such as capping protein or chromosomes), it is stabilized and will not be disassembled.
    • This way allows microtubules to construct certain permanent structures in cells.
• Microtubules organize organelles:
  • Microtubules basically construct a network in the cytosol for organelles to anchor.
• Dynein and Kinesin are two motor proteins that perform polar transport along microtubules:
  • Each of the Dynein and Kinesin molecules consist of a motor head and a cargo tail.
    • Cargo tails carry cargo and motor heads slid along the microtubules at respective directions.
    • The major functional difference between Dynein and Kinesin is the transport directions, kinesin moves toward to plus end, and dyneins move toward to minus end.
  • Dynein and Kinesin walk toward opposite directions
• Polarity of microtubules:
  • Most of the animal cells are polarized.
    • One end of the cell is functionally different from the other end.
      • This provides directions in the “ocean” of cytoplasm for transport.
  • Microtubules often serve as “tracks,” allowing motor proteins to transport “goods” along these tracks.
    • The polarity of microtubules provides directions for transport to follow.
  • Transport along axons is an example (this is not the action potential nor neuronal signals).
• Flagella are made of microtubules:
  • Flagella are made of 9+2 bundles of microtubules.
    • Each bundle is composed of two microtubules, one attached with the cargo tail of the dynein.
      • When dynein arm (motor head) pushes neighbor bundles, two bundles passing each other.

Cytoskeleton: Actin Filaments

• Actin molecules are the subunits of the Actin Filament
• Myosin attaches and walks on the actin filament to perform movement
• Contractile Rings contain actin to perform contraction
• Cell surface is reinforced by Cell Cortex:
  • Cell Cortex is a surface protein meshwork that determines the shape of the cells.
  • Actin is an important part of cell cortex network.
• Actin filaments are composed of actin molecules:
  • All actin molecules (subunits) connected in the same direction.
    • Every subunit is identical to others (unlike microtubules which have \alpha and \beta tublins).
  • Two strands of actin filaments combine together to be a two-stranded helix.
• Actin filaments polymerization (adding subunits to make it longer) is similar to microtubules:
  • When an ATP binds to actin, the actin is incorporated into the growing strand.
  • When the ATP is hydrolyzed to ADP (lost energy). The stability is decreased, and subunits fall apart.
• Assembly of Actin meshwork to push the membrane:
  • The polymerization of actin filament is stopped when a capping protein attaches and protects the plus end.
  • APR complex (a complex of proteins) attaches to the existing actin filament to provide a site of attachment.
  • The new filament (branch) starts building on this site.
  • The hydrolyzed ADP attaching to actin will promote the depolymerization of the filament by Depolymerizing Protein.
• When the polymerization adds length to actin filaments, the filaments elongated and push the membrane and make the cell to crawl.
• Myosin “walks” on actin filaments to perform cellular transport (Myosin I):
  • With the binding-releasing actions and conformational changes, myosin can “walk” along the long actin filaments, from minus end to plus end.
    • (Dynein and Kinesin almost always associate with Microtubules)
  • This one-way walking ensures the movement of myosin-associated organelles proceeds in a precise direction.
• Myosin action in muscle (Myosin II):
  • The head contains an ATPase which hydrolyzes ATP to get energy.
    • When the head hydrolyzes an ATP, it starts pulling actin.
  • Muscular cells are filled with Actin and Myosin fibers:
    • Since they fill the cell up so tightly, nuclei and other organelles are pushed aside.
    • Each myofibril is filled with actin and myosin filaments.
• How do Actin and Myosin work on contraction?
  • The head is locked to the actin filament when it is rest.
  • When the head releases ADP, It also pulls actin back.
  • At the end of the cycle, the head is locked to the actin filament again.
  • When the ATP binds to the head, the head releases actin.
  • When the head hydrolyzes ATP, it also moves forward along the actin filament.
  • When the head releases phosphate, it binds to a location further ahead (the head has moved forward).
  • Muscular cells are composed of Sarcomeres with Ca^{2+} channels on their membrane.
    • When these channels open, the Ca^{2+} flows into the cytoplasm (the location of actin and myosin) of sarcomeres to trigger the contraction.
      • The cylinder unit is called Sarcomere
  • Tropomyosin proteins normally cover the myosin binding sites on the actin to prevent myosin from binding and pulling the actin.
    • When Ca^{2+} ions flow in, they will bind to troponin, another protein that is with tropomyosin.
      • This removes tropomyosin from actin
      • Once they are released, the binding sites are free for myosin to bind.