Cell Motility: Microtubule- and Actin-Based Movement in Eukaryotic Cells

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Cell Motility: Overview

Cell motility encompasses the diverse mechanisms by which cells and their internal components move. This includes movement at the subcellular, cellular, and tissue levels, driven primarily by cytoskeletal elements and associated motor proteins.

Subcellular motility: Separation of chromosomes during cell division, intracellular transport of vesicles and organelles.
Cellular motility: Cilia/flagella-based movement, amoeboid movement, chemotaxis, and tumor metastasis.
Tissue-level motility: Muscle contraction, resulting from the coordinated action of many muscle cells.

Microtubule-Based Motility

Motor Proteins and Microtubules

Microtubule-based motility is powered by motor proteins that convert chemical energy from ATP hydrolysis into mechanical work, enabling the transport of cellular cargo along microtubule tracks.

Kinesins: Move toward the plus end of microtubules, typically transporting cargo away from the cell center.
Dyneins: Move toward the minus end of microtubules, often transporting cargo toward the cell center.

Table of selected motor proteins of eukaryotic cells

Kinesin Structure and Function

Kinesin-1 is a well-studied motor protein with two globular heads (motor domains), a coiled-coil stalk, and a light-chain tail that binds cargo. The heads contain microtubule-binding and ATPase sites, generating force through ATP hydrolysis.

Diagram of kinesin structure

Kinesin Movement Mechanism

Kinesin moves along microtubules in a processive, stepwise manner, with each step corresponding to the hydrolysis of one ATP molecule and a displacement of approximately 8 nm. The movement is often described as a "walking" motion, where one head detaches and moves forward to bind a new tubulin subunit.

Diagram of kinesin walking mechanism

Hand-Over-Hand vs. Inchworm Models

Two models have been proposed for kinesin movement: the hand-over-hand and inchworm mechanisms. Experimental evidence supports the hand-over-hand model, where the two heads alternate in leading, resulting in 8-nm steps per ATP hydrolyzed.

Hand-over-hand and inchworm models of kinesin movement

Dynein Structure and Function

Cytoplasmic dynein is a large, multi-subunit complex with heavy, intermediate, and light chains. The heavy chains bind to microtubules and generate movement toward the minus end, while the light chains interact with the dynactin complex, which links dynein to cargo membranes.

Diagram of cytoplasmic dynein and dynactin complex

Microtubule-Based Vesicle Transport

Microtubules serve as tracks for the transport of vesicles and organelles within cells. Kinesin and dynein mediate anterograde and retrograde transport, respectively, ensuring the proper distribution of cellular components.

Diagram of vesicle transport by kinesin and dynein on microtubules

Cilia and Flagella: Structure and Function

General Features

Cilia and flagella are motile appendages of eukaryotic cells, composed of a core axoneme connected to a basal body. Cilia are typically short and numerous, while flagella are longer and fewer in number.

SEM of cilia on a mammalian trachea cell

Axoneme Structure

The axoneme is the core structure of cilia and flagella, characterized by a "9+2" arrangement of microtubules: nine outer doublets surrounding a central pair. Dynein arms attached to the A tubule of each doublet interact with the B tubule of the adjacent doublet, generating sliding forces that are converted into bending motions by nexin links and other cross-linking proteins.

Diagram and cross-sections of cilia and axoneme structure Cross-section of axoneme showing 9+2 microtubule arrangement

Mechanism of Ciliary and Flagellar Movement

Dynein arms "walk" along adjacent microtubule doublets, causing them to slide against each other. This sliding is constrained by cross-linking proteins, resulting in the bending of the entire axoneme and the characteristic beating motion of cilia and flagella.

Dynein walking on microtubules generates sliding force Diagram showing how cross-linking restricts sliding and causes bending

Actin-Based Motility and Muscle Contraction

Myosin Structure and Function

Myosins are actin-based motor proteins with a head domain (containing actin-binding and ATPase sites), a neck region (binding light chains), and a variable tail domain. Myosin II forms bipolar filaments in muscle cells, enabling contraction.

Organization of Skeletal Muscle Tissue

Skeletal muscle is composed of bundles of myofibers (multinucleated muscle cells), which contain myofibrils made up of repeating sarcomeres—the fundamental contractile units. Sarcomeres are defined by Z lines and contain thick (myosin) and thin (actin) filaments.

Structure of the Sarcomere

Thick filaments: Composed of myosin II, bundled together by myomesin.
Thin filaments: Composed of actin, attached to Z-disks, and decorated by tropomyosin and troponin. Capping proteins (Cap Z and tropomodulin) regulate filament length.

Major Protein Components of Vertebrate Skeletal Muscle

The following table summarizes the major protein components and their functions in vertebrate skeletal muscle:

Protein	Function
G-actin	Major component of thin filaments
Myosin	Major component of thick filaments
Tropomyosin	Binds along thin filaments
Troponin	Calcium regulation of contraction
Titin	Links thick filaments to Z line
Nebulin	Stabilizes thin filaments
Myomesin	Myosin-binding protein at M line
α-actinin	Bundles actin filaments at Z line
Ca2+ ATPase	Transports Ca2+ into SR for relaxation
CapZ	Caps actin at Z line
Tropomodulin	Maintains thin filament length

Sliding Filament Model of Muscle Contraction

Muscle contraction occurs as thick and thin filaments slide past each other, increasing their overlap. The I band shortens, Z lines are pulled together, and the sarcomere contracts. The process is driven by the cyclic interaction of myosin heads with actin filaments, powered by ATP hydrolysis.

Each myosin head binds to actin, undergoes a power stroke, and releases upon ATP binding.
ATP hydrolysis "cocks" the myosin head for the next cycle.

Regulation of Muscle Contraction

In striated muscle, contraction is regulated by the troponin-tropomyosin complex. In the absence of Ca2+, tropomyosin blocks myosin binding sites on actin. When Ca2+ binds to troponin C, tropomyosin shifts, exposing binding sites and allowing contraction.

Ca2+ increase: Binds troponin C, enables contraction.
Ca2+ decrease: Troponin-tropomyosin blocks myosin, muscle relaxes.

Smooth Muscle and Non-Muscle Cell Motility

Smooth muscle contraction is regulated differently, involving calmodulin and myosin light chain kinase (MLCK), rather than troponin. Actin-based motility in non-muscle cells includes cell crawling via lamellipodia and filopodia, driven by actin polymerization and myosin activity.

Chemotaxis

Chemotaxis is the directed movement of cells in response to chemical gradients, involving cytoskeletal rearrangements and actin-based protrusions.

Summary Table: Motor Proteins and Their Functions

Molecules	Typical Function
Cytoplasmic dynein	Motion toward minus end of microtubule
Axonemal dynein	Activation of sliding in flagellar microtubule
Kinesins	Motion toward plus end of microtubule
Myosin I, monomer	Motion along actin filaments
Myosin II, filament	Slides actin filaments in sarcomere of muscle

Table of selected motor proteins of eukaryotic cells

Key Concepts and Review Questions

All motor proteins convert chemical energy (ATP) into motion.
All myosins are motor proteins that bind to and exert force on microfilaments (actin).
Calcium regulation of muscle contraction occurs via conformational changes in tropomyosin, mediated by troponin binding Ca2+.