TR

Cytoskeleton

Introduction

  • The eukaryotic cytoskeleton: a network of filaments and tubules that extends from the nucleus to the plasma membrane.
  • The cytoskeleton contains three types of elements responsible for cell shape, movement within the cell, and movement of the cell: 
    • Actin filaments (or microfilaments)
    • Microtubules
    • Intermediate filaments

Microtubules

  • Hollow tubes

  • Wall consists of 13 columns of tubulin

  • Maintain cell shape

  • Cell motility

  • Chromosome movement

  • Organelle movement

  • Microtubule: a stiff hollow cylinder about 24 nm in diameter

  • Joined end-to-end to form protofilaments, with alternating α & β subunits

  • Staggered assembly of 13 protofilaments yields a helical arrangement of tubulin heterodimers in the cylinder wall

    • GTP must be bound to both α and β subunits for a tubulin heterodimer to associate with other heterodimers to form a protofilament

  • Microtubules must have polarity

    • Plus and minus ends
    • Plus ends → more dynamic where both growth and shrinkage are fast
    • Minus end → slow growing end

  • Important concepts to microtubule assembly:

    • Nucleation
    • The start of microtubule assembly
    • Rate limiting step in microtubule assembly
    • Treadmilling
    • Recycling of tubulin dimers
    • Subunits are recruited at the plus end and shed from the minus end at an identical rate
      • Allows the microtubule to maintain a constant length
    • Dynamic Instability
    • Essential characteristic to microtubule function

Microfilaments

  • 2 intertwined strands of actin
  • Maintain cell shape
  • Changes in cell shape
  • Muscle contraction
  • Cytoplasmic streaming
  • Cell motility
  • Cell division
  • Microfilaments: solid rods about 7 nm in diameter, built as a twisted double chain of actin subunits
  • The structural role of microfilaments is to bear tension, resisting pulling forces within the cell
  • They form a 3-D network called the cortex just inside the plasma membrane to help support the cell’s shape
  • Bundles of microfilaments make up the core of microvilli of intestinal cells

Intermediate Filaments

  • Fibrous proteins supercoiled into thicker cables
  • Anchorage of nucleus and other organelles
  • Formation of nuclear lamina
  • Cytoplasmic
    • Keratin in epithelial cells
    • Vimentin in connective and muscle cells
    • Neurofilaments
  • Nuclear
    • Nuclear lamina

General Functions of the Cytoskeleton

  • Mitosis
    • Pulls chromosomes apart 
  • Cytokinesis
  • Drives and guides intracellular traffic of organelles
  • Maintains organelle positions
  • Moves material from one part of the cell to another
  • Supports plasma membrane and gives shape to the cell
  • Cell motility (sperm) and movement (muscle contraction)