Topic 9

• Cytoskeleton


  A structure that helps cells maintain their shape and internal organization, and it also provides mechanical support that enables cells to carry out essential functions like division and movement

  
• Cytoskeletal filaments


  • Microfilaments (Actin filaments)
  • Microtubules
  • Intermediate filaments

  
• Actin


  The most abundant cytoskeletal protein of most cells, which polymerizes to form actin filaments

  
• Actin filaments


  • Particularly abundant beneath the plasma membrane, where they form a network that provides mechanical support, determines cell shape, and allows movement of the cell surface, thereby enabling cells to migrate, engulf particles, and divide

  
• Globular proteins


  Proteins with a spherical shape that are soluble in water, acids, and bases
• Fibrous proteins


  Long, strand-like proteins that are insoluble in water, weak acids, and weak bases
• Actin filaments


  • Thin, flexible fibers approximately 7 nm in diameter and up to several μm in length
  • Have a distinct polarity and their ends (called plus ends and minus ends) are distinguishable from one another

  
• Actin polymerization


  1. Nucleation

  2. Elongation

  3. Steady State (or Stationary Phase)

  
• Nucleation


  The first step in actin polymerization, which is the formation of a small cluster consisting of three actin monomers

  
• Elongation


  Actin filaments are able to grow by the reversible addition of monomers to both ends, but the plus end elongates up to ten times faster than the minus end

  
• Steady state


  The polymerization rate at the plus end is equal to that of the depolymerization rate from the minus end
• Treadmilling


  The continuous addition of actin monomers at the plus end and dissociation from the minus end, resulting in the progression of G-actin monomers from the plus to the minus end

  
• Actin treadmilling is crucial to several functions in eukaryotic cells, such as cell motility and migration, endocytosis, and exocytosis
• Treadmilling is responsible for up to 50% of energy consumption in some cells
• Actin-binding proteins


  A diverse group of proteins that interact with actin filaments and regulate their assembly and disassembly

  
• Formin


  • A family of actin-binding proteins that nucleate the initial polymerization of actin monomers and then move along the growing filament, adding new monomers to the plus end

  
• Arp2/3 complex


  • Binds ATP-actin near the plus ends of filaments and initiates the formation of branched actin filaments

  
• Tropomyosins


  Rod-like dimers that form head-to-tail polymers along the length of actin filaments and regulate the access of actin-binding proteins to the filaments

  
• Capping proteins


  Regulate actin polymerization by binding to the end of an actin filament, which blocks the addition and loss of actin subunits

  
• Motor proteins


  Molecular motors associated with the cytoskeleton that use the energy derived from repeated cycles of ATP hydrolysis to move steadily along cytoskeletal filaments
• Motor proteins


  • Carry membrane-enclosed organelles to their appropriate locations in the cell
  • Generate the force that drives phenomena like muscle contraction, ciliary beating, and cell division
• Myosin


  A motor protein often associated with actin filaments and responsible for many types of cell movement, including muscle contraction and the intracellular transport of vesicles and organelles

  
• Myosin


  Myosin is a motor protein that is often associated with muscle contraction and is responsible for many types of cell movements. Interactions of actin and myosin are responsible for muscle contraction and also for a variety of movements of non-muscle cells, including the intracellular transport of vesicles and organelles.

  
• Myosin


  • The type of myosin present in muscle (myosin II) is a large protein consisting of two identical heavy chains and two pairs of light chains. Each heavy chain consists of a globular head region and a long α-helical tail

  
• Mechanism that powers myosin's activity

  The activity of myosin as a molecular motor is powered by its ability to bind and hydrolyze ATP, which provides the energy that drives the movement. This translation of chemical energy to movement is mediated by changes in the shape of myosin resulting from ATP binding.

  
• Direct evidence for the motor activity of the myosin head
• Skeletal muscles


  • Skeletal muscles are bundles of muscle fibers, which are single large cells (~50 μm in diameter and up to several centimeters in length) formed by the fusion of many individual cells during development. Most of the cytoplasm consists of myofibrils, which are cylindrical bundles of two types of filaments: thick filaments of myosin and thin filaments of actin. Each myofibril is organized as a chain of contractile units called sarcomeres, which are responsible for the striated appearance of skeletal and cardiac muscle.

  
• Thick filaments of muscle

  • The thick filaments of muscle consist of several hundred myosin molecules associated in a parallel staggered array by interactions between their tails. The globular heads of myosin bind actin, forming cross-bridges between the thick and thin filaments.

  
• A molecular model for myosin function has been derived both from the three-dimensional structure of myosin and from in vitro studies of myosin movement along actin filaments

  
• Myosin action cycle


  The cycle starts with myosin (in the absence of ATP) tightly bound to actin. ATP binding dissociates the myosin–actin complex and the hydrolysis of ATP then induces a conformational change in myosin. This change affects the neck region of myosin that binds the light chains, which acts as a lever arm to displace the myosin head by about 5 nm. The products of hydrolysis (ADP and Pi) remain bound to the myosin head, which is said to be in the "cocked" position. The myosin head then rebinds at a new position on the actin filament, resulting in the release of Pi. This triggers the "power stroke" in which ADP is released and the myosin head returns to its initial conformation, thereby sliding the actin filament toward the M line of the sarcomere.

  
• Cell surface protrusions and cell movement

  • The movement of cells across a surface represents a basic form of cell locomotion employed by a wide variety of different kinds of cells. Examples include the crawling of amoebas, the migration of embryonic cells during development, the invasion of tissues by white blood cells to fight infection, the migration of cells involved in wound healing, the spread of cancer cells during metastasis, and the extension of nerve cell processes during the development of the nervous system. All of these movements are based on local extensions of the plasma membrane that extend from the leading edge of a moving cell.
• Cell surface projections


  • Pseudopodia
  • Lamellipodia
  • Filopodia
• Pseudopodia


  Projections of the cytoplasm responsible for the movement of amoebas across a surface or phagocytosis. Actin filaments are the structural components that generate the force needed for pseudopodia extension and retraction.

  
• Phagocytosis


  A cellular process for ingesting and eliminating particles, including foreign substances, microorganisms, and dead cells. Phagocytes are a type of white blood cell that play an important role in the immune system's defense against pathogens and foreign particles.
• Lamellipodia


  Broad, sheetlike extensions at the leading edge of fibroblasts, which contain a network of actin filaments.

  
• Filopodia


  Very thin projections of the plasma membrane, supported by actin bundles, that extend from lamellipodia.

  
• How do pseudopodia, lamellipodia and filopodia form?

  The formation and retraction of these structures during cell movement are based on the regulated assembly and disassembly of actin filaments.

  
• Cell migration steps


  Extension of protrusions such as pseudopodia, lamellipodia, or filopodia to establish a leading edge of the cell. 2) Attachment of these extensions to the substratum across which the cell is moving. 3) Dissociation of the trailing edge of the cell from the substratum and retraction into the cell body.
• Microtubules are composed of a single type of globular proteins called tubulins.

  
• Structure of microtubules


  • The building blocks of microtubules are tubulin dimers consisting of two closely related polypeptides, each comprising about 450 amino acids: α-tubulin and β-tubulin. Tubulin dimers polymerize to form microtubules, which generally consist of 13 linear protofilaments assembled around a hollow core.