Lecture 22

• Myosins


  Actin-binding motor proteins found in eukaryotic cells
• Myosin structure


  • Consist of heavy chains and light chains
• Myosin functions


  Muscle contractions, cell motility, intracellular transport
• Myosin heavy chains


  • Contain motor domain for ATP hydrolysis & actin binding
• Myosin light chains


  • Regulate myosin activity & localization within the cell
• Myosin types in the myosin superfamily

  • Myosin I - involved in membrane and vesicle transport
  • Myosin II - Forms bipolar filaments, helps with muscle contractions, & cytokinesis
  • Myosin V - Intracellular transport, moving vesicles and organelles along actin filaments
  • Myosin VI - Functions in endocytosis and intracellular transport, moving cargo towards the minus end of actin filaments
• Myosin II functions


  Responsible for muscle contraction in animals and cytokinesis (cell division) in non-muscle cells
• Muscle contraction by myosin II

  1. Myosin II forms a thick filament that interacts with actin filaments

  2. ATP hydrolysis by myosin II powers the movements of actin filaments [relative to myosin filaments]

  3. Causing muscle contraction
• Cytokinesis by myosin II


  1. Myosin II is vital to the contraction of the contractile ring

  2. The contraction of the ring pinches the cell membrane inward causing the separation of two daughter cells
• Skeletal muscle tissue


  • Composed of many, long muscle cells (muscle fibers) arranged in parallel
  • Responsible for generating force and movement
• Muscle fibers


  • Inside each muscle fiber are numerous myofibrils arranged in parallel along the long axis of the muscle cell
  • Responsible for the muscles striated appearance
• Myofibrils


  • Each myofibril consists of many sarcomeres (the fundamental unit of contraction) arranged end-to-end
  • Contain contractile proteins actin and myosin, which interact to produce muscle contractions
• Sarcomere components


  • Thin filaments: actin plus actin-binding proteins
  • Thick filaments: bipolar myosin II filaments
• Excitation-contraction coupling


  1. Motor neuron depolarizes the muscle cell

  2. T-tubules bring the depolarization signal deep within the myofibrils

  3. Depolarization opens Ca2+ channels in smooth ER

  4. Regulation of contraction by Ca2+ in skeletal muscle

  5. Relaxation of muscles requires ATP
• Microtubule structure


  • Most microtubules are ~25 nm hollow tubes composed of 13 protofilaments arranged side-by-side
  • Fundamental subunit: αβ-tubulin heterodimer forms as soon as the α and β subunits are synthesized
  • Both subunits share a similar structure
  • Polarity of a protofilament: the minus-end starts with an α and the plus-end ends with a β-subunit
• Microtubule polymerization


  Differs in vitro vs. in vivo
• Microtubule polymerization


  • αβ- heterodimers have high-affinity for other tubulin heterodimers when the β-subunit is bound to GTP. They lose affinity for each other when GDP is bound to the β-subunit
  • Both assembly and disassembly are fastest at the (+) end
• Microtubule-organizing Centers (MTOCs)


  Cellular structures that initiate the nucleation and outgrowth of microtubules
• Centrosome


  • The primary MTOC in most cells, from which cytoplasmic microtubules arise
• Microtubule-Organizing Centers (MTOCs)


  Specialized structures within cells that initiate the nucleation and outgrowth of microtubules
• MTOCs serve as sites where microtubules originate and grow
• MTOCs


  • They are vital for organizing the microtubule cytoskeleton, which is essential for various cellular processes such as cell division, intracellular transport, and cell shape maintenance
• Centrosome


  The primary MTOC in most cells, from which cytoplasmic microtubules arise
• Centrosome


  • It is a cellular organelle composed of 2 centrioles surrounded by pericentriolar material (PCM)
  • It plays a central role in organizing the microtubule network and coordinating various cellular processes, including cell division and cell polarity
• Basal bodies


  The MTOCs for the assembly and organization of microtubules in flagella and cilia
• Basal bodies


  • They are structurally similar to centrioles
  • They are involved in cellular locomotion, sensory functions, and fluid movement in various organisms
• Centrosome structure


  • It is composed of two centrioles arranged perpendicular to each other and surrounded by a cloud of pericentriolar material (PCM)
  • The centrioles are cylindrical structures consisting of microtubule triplets arranged in a 9+0 pattern
  • The PCM contains various proteins involved in microtubule nucleation, anchoring, and organization
• Kinesin


  A family of motor proteins that move across the microtubules in the + end, using energy from ATP hydrolysis to power their movement
• Kinesins


  • They are involved in various cellular processes, including intracellular transport of vesicles, organelles, and protein complexes
• Dynein


  A family of motor proteins that move across the microtubules in the - end, utilizing ATP hydrolysis to generate the force required for movement
• Dyneins


  • They are involved in a variety of cellular processes, including retrograde transport of vesicles and organelles toward the cell center, as well as the positioning and organization of cellular structures such as the mitotic spindle during cell division
• Microtubule-associated stabilizing proteins


  A diverse group of proteins that bind to microtubules and regulate their dynamics by promoting stability and inhibiting disassembly
• Microtubule-associated stabilizing proteins


  • They play essential roles in regulating microtubule dynamics, cytoskeletal organization, and cellular functions such as cell shape maintenance and neurite outgrowth
• Intermediate filaments


  • They perform primarily a structural role: reinforce and protect cells and organize them into tissues (cell-to-cell contacts) e.g. skin, hair, and fingernails
  • They provide mechanical support for the plasma membrane, where it contacts other cells and the extracellular matrix
  • They play no role in cell motility or organelle transport
• Intermediate filaments


  They have a diameter ranging from approximately 8 to 12 nanometers, making them intermediate in size between microfilaments (actin filaments) and microtubules
• Intermediate filaments


  • They form braided rope-like fibers, not tubes (microtubules) or beaded-strings (microfilaments)
  • They do not bind nucleotides such as ATP or GTP, and are instead composed of fibrous proteins that self-assemble to form filaments
  • They are highly stable and do not exhibit dynamic instability like microtubules
  • They play no role in directed movements such as cell migration or intracellular transport
• Intermediate filament monomers


  • They have a central α-helical rod domain flanked by non-helical N-terminal and C-terminal domains
  • The α-helical rod domain is the structural core and responsible for mediating interactions between monomers during filament assembly
• Intermediate filament assembly


  1. Monomers dimerize as parallel fibers (coiled-coil dimmer)

  2. Tetramers form as two, staggered anti-parallel dimers

  3. Two protofilaments associate side-to-side to make protofibrils

  4. Four protofibrils are braided together to compose the final intermediate filaments
• Types of intermediate filaments

  • Keratins
  • Vimentin and vimentin-related
  • Neurofilaments
  • Nuclear lamins
• Keratins


  • They are a diverse group of intermediate filament proteins found in epithelial cells (tissue like skin, hair, and nails)
  • They provide mechanical strength and structural integrity