TheCell7e Ch13 Lecture

The Cytoskeleton and Cell Movement

Introduction

  • The cytoskeleton is a network of protein filaments in eukaryotic cells.

  • It provides structural support, determines cell shape, positions organelles, and aids in cytoplasmic organization.

  • Responsible for cell movement and internal transport of organelles.

  • It is dynamic, continuously reorganizing as cells move and change shape.

  • Composed of three main types of protein filaments:

    • Actin filaments (microfilaments)

    • Microtubules

    • Intermediate filaments

Structure and Organization of Actin Filaments

  • Actin polymerizes to form actin filaments, which are flexible, 7 nm in diameter, and several μm in length.

  • Organized into structures like bundles and 3-D networks.

  • Actin-binding proteins regulate assembly, disassembly, and interactions with other cell structures.

Examples of Actin-Binding Proteins:

  • Monomer binding: Profilin, twinfilin

  • Filament initiation: Arp2/3, formin

  • End capping: CapZ, tropomodulin

  • Stabilization: Nebulin, tropomyosin

  • Cross-linking: a-actinin, filamin, fimbrin, villin

  • Actin linkage: Dystrophin, spectrin, talin, vinculin

  • Severing: Cofilin, gelsolin

Actin Characteristics

  • First isolated from muscle cells in 1942, comprises 5–10% of total protein in eukaryotic cells.

  • Mammals have six actin genes; four expressed in muscle cells, two in nonmuscle cells.

  • Highly conserved; yeast actin is 90% identical to mammalian actin.

Assembly and Dynamics of Actin Filaments

  • 3-D structure of actin molecules determined in 1990; each G-actin has tight binding sites for polymerization into F-actin.

  • Polarity: All actin monomers oriented in the same direction; important for assembly and myosin movement.

  • Nucleation: Initial step of polymerization; dimers and trimers form before monomer addition.

  • Treadmilling: Actin polymerization dynamic; barbed end grows faster than pointed end, important for cellular regulation.

    • ATP-actin adds to barbed end, ADP-actin dissociates from pointed end.

Regulation of Actin Dynamics

  • Various drugs affect actin polymerization (e.g., cytochalasins, phalloidin).

  • Formins nucleate initial polymerization of long unbranched filaments.

  • The Arp2/3 complex drives growth of branched actin filaments, crucial for cell movement.

Actin Structures

  • Organized into bundles (parallel arrays) and networks (3-D meshworks).

  • Cross-linking proteins maintain filament organization, influencing cell movements and shape.

  • Specialized structures in epithelial cells like microvilli enhance surface area through parallel actin bundles.

Myosin Motors

  • Myosin: a molecular motor that converts ATP's chemical energy into mechanical energy.

  • Structure of muscle fibers consists of myofibrils that contain thick myosin and thin actin filaments.

  • Sliding filament model: During contraction, myosin heads bind to actin, allowing filaments to slide past one another, shortening the muscle.

  • Myosin II: Contains heavy and light chains; movement is driven by ATP hydrolysis.

Microtubules

  • Rigid, dynamic hollow rods made from tubulin dimers forming protofilaments.

  • Exhibit polarity; play roles in cell shape and movement.

  • Microtubules undergo cycles of assembly and disassembly controlled by GTP hydrolysis.

Microtubule Motors and Movement

  • Two main families of motor proteins:

    • Kinesins (toward plus end)

    • Dyneins (toward minus end)

  • Kinesin I has directed movement along microtubules while dynein transports cargo in the opposite direction.

Intermediate Filaments

  • Provide mechanical strength, composed of various proteins (e.g., keratins, vimentin).

  • More stable than actin and microtubules, do not have dynamic assembly.

  • Important in maintaining structural integrity under mechanical stress.