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)

\