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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)

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)