Cytoskeleton: Filaments and Their Building Blocks

LECTURE 30 (A.L. Kasinski)

BIOL231 The Cytoskeleton: Filaments and Their Building Blocks

Introduction to Cytoskeleton

• Eukaryotic Cell as an "Endoskeleton"

  • The eukaryotic cells contain a cytoskeleton that underpins mechanical properties necessary for shape, movement, and organization of internal components.

  • Questions addressed:

  • What maintains a cell's shape?

  • What organizes the cytoplasmic components?

  • How does a cell move?

• Definition of Cytoskeleton

  • The cytoskeleton consists of a framework of protein filaments that:

  1. Provides structural support.

  2. Resists or generates tension, compression, and deformation.

  3. Serves as a docking site for proteins and organelles.

  4. Acts as tracks for organelle movement.

  5. Generates force to change cell shape and enable locomotion.

Three Filament Systems of the Cytoskeleton

• Overview of Filament Systems

  • The cytoskeleton is composed of three distinct filament systems:

  1. Intermediate Filaments (IFs)

  2. Microtubules (MTs)

  3. Actin Filaments

  • Most vertebrate cells contain all three systems.

  • Notable dimensions:

  • Actin filaments: ~7 nm in diameter.

  • Intermediate filaments: ~10 nm in diameter.

  • Microtubules: ~25 nm in diameter with a helical pattern.

  • Formation of all three filaments is due to non-covalent association of subunit proteins.

Physical Properties of Cytoskeletal Filaments

A. Intermediate Filaments (IFs)

• Characteristics

  1. Resistance to Disruption

  • IFs are the most resistant to harsh conditions (heat, detergent, high salt).

  1. Tensile Strength

  • IFs have the highest tensile strength among the three systems, enabling resistance to mechanical stresses, particularly shearing stress.

  1. Network Formation

  • Form a network throughout the cytoplasm, interacting with cell-cell junctions, and surrounding the nucleus.

  1. Nuclear Lamina

  • A meshwork of IFs in the nucleus supports the nuclear envelope.

  1. Subunits

  • Family of related IF subunit proteins; different proteins expressed in different cell types.

  • Elongated, fibrous proteins with a high percentage of alpha-helix structure, with globular N-terminal and C-terminal regions connected by an extended alpha-helical region.

  1. Assembly and Structure

  • Monomers form stable coiled-coil dimers, which further combine into tetramers.

    • Important: dimers are staggered, aligning N and C termini oppositely, critical for filament stability.

  • Tetramers align end-to-end into strands, forming a final filament structure by twisting together eight strands.

  • Key Property: IFs are symmetric; both ends of IFs are structurally identical.

B. Microtubules (MTs)

• Characteristics

  1. Structure and Dynamics

  • Tubular structures with a hollow interior, 25 nm in diameter, can be tens to hundreds of $\mu$m long.

  • Rapidly assemble and disassemble, especially dynamic through the cell cycle.

  1. Specialized Structures

  • Form larger structures like cilia and comprise the main part of mitotic and meiotic spindles.

  • Typically anchored at one end to a centrosome.

  1. Subunit Composition

  • Built from $\alpha$- and $\beta$-tubulin subunits, approximately 50 kDa in size.

  • Tubulin exists as heterodimers of $\alpha$-tubulin and $\beta$-tubulin linked by strong non-covalent bonds.

  1. Assembly into Microtubules

  • 13 parallel columns of dimers, or protofilaments, making up the microtubule wall.

  • The wall exhibits a helical arrangement of dimers, promoting structural polarity where one end features exposed $\alpha$-tubulin and the other $\beta$-tubulin.

C. Actin Filaments

• Characteristics

  1. Structural Features

  • The thinnest and most flexible, about 7 nm in diameter. Responsible for cell shape and movement:

    • Bundled for protrusions, form sheet-like networks, and contractile rings during cell division.

  D. Assembly and Nucleotide Hydrolysis

  1. Actin Monomer

     • The building block is a globular protein, displaying a distinct polarity (G-actin). Each G-actin monomer contains a nucleotide-binding cleft ($ATP$ or $ADP$).

  2. Assembly and Polarity

     • G-actin monomers polymerize in a head-to-tail fashion to form a two-stranded helix (F-actin). This head-to-tail arrangement confers structural polarity to the filament, with a fast-growing "plus" end and a slow-growing "minus" end. $ATP$ hydrolysis within the filament contributes to its dynamic instability.