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)