Cellular Movement: Motility and Contractility – Study Notes

Cellular Movement: Motility and Contractility

Introduction to Cell Motility and Contractility

Cellular movement is a fundamental process in biology, involving the motion of cells and their internal components. This movement is essential for various physiological functions, including development, immune responses, and tissue repair.

• Cell motility refers to the movement of a cell or organism through its environment, or the movement of the environment past or through a cell.

• Contractility describes the shortening of muscle cells, a specialized form of motility.

Mobile Systems in Cells

Types and Levels of Motility

Motility occurs at multiple biological levels, including tissues, cells, and subcellular structures. Intracellular components move to facilitate essential processes such as cell division and transport.

• Microtubules and microfilaments are cytoskeletal elements that provide tracks for movement.

• Motor proteins interact with these filaments to produce motion at the molecular level.

Major Eukaryotic Motility Systems

Microtubule-Based vs. Microfilament-Based Motility

Eukaryotic cells utilize two primary systems for movement: microtubule-based and microfilament-based motility.

• Microtubule-based motility: Involves fast axonal transport in neurons and the sliding of microtubules in cilia and flagella.

• Microfilament-based motility: Exemplified by muscle contraction.

Motor Proteins and Microtubule-Based Movement

Kinesins and Dyneins

Motor proteins such as kinesins and dyneins move along microtubules, transporting cellular cargo and facilitating movement.

• They couple ATP hydrolysis to changes in shape and movement.

• They undergo cycles of ATP hydrolysis, ADP release, and acquisition of new ATP.

• They share structural features and can move along microtubules, which is especially important for microtubule-based motors.

Microtubule-Associated Motor Proteins

Kinesins and dyneins move cargo along microtubules, providing the force needed for movement within cells.

• Kinesins generally move toward the plus end of microtubules (anterograde transport).

• Dyneins move toward the minus end (retrograde transport).

Fast Axonal Transport

Proteins and organelles are transported along axons via microtubule tracks, a process essential for neuronal function.

• Kinesin I is responsible for anterograde transport (toward the synapse).

• Cytoplasmic dynein is responsible for retrograde transport (toward the cell body and MTOC).

• Transport rates can reach about 2 μm/sec.

Kinesin Structure and Function

Kinesins are dimeric proteins with two heavy chains and two light chains. The heavy chains contain globular domains that bind microtubules and a coiled-coil stalk, while the light chains are associated with the tail.

• Kinesin movement resembles "walking," with the two globular head domains taking turns as the front foot.

• Kinesins exhibit processivity, moving long distances along microtubules before detaching.

Kinesin Families

• Kinesins are classified by amino acid sequence into families; some are homodimers, others heterodimers.

• Kinesin-14 is minus-end directed; kinesin-5 is bidirectional.

• Kinesin-13 aids in microtubule depolymerization (catastrophins).

Dyneins

Dyneins are found in axonemes and the cytosol. Cytoplasmic dyneins associate with the protein complex dynactin to link cargo to kinesin. Axonemal dyneins are involved in ciliary and flagellar movement.

Microtubule Motors and Vesicle Transport

Role in Endomembrane System

Microtubule motors are crucial for shaping and transporting vesicles within the endomembrane system.

• Extensions of the endoplasmic reticulum (ER) and vesicles from the ER to the Golgi are moved by microtubule motors.

Cilia and Flagella: Motile Appendages

General Features

Cilia and flagella are motile appendages found on eukaryotic cells, sharing a common structural basis for movement.

• Cilia are about 2–10 μm long and occur in large numbers on cell surfaces.

• Flagella are longer (up to 200 μm) and usually fewer per cell.

• Cilia beat in a coordinated pattern, generating a force parallel to the cell surface.

• Flagella move cells through fluid environments, working like a whip.

Axoneme and Basal Body Structure

Cilia and flagella have a core structure called the axoneme, connected to a basal body (similar to a centriole).

• The axoneme has a "9 + 2" arrangement: nine outer doublets and two central microtubules.

• Basal bodies have nine sets of triplet microtubules.

Primary Cilia

Primary cilia are sensory structures with a "9 + 0" arrangement, lacking the central pair. They are important in development and sensory functions.

• Defects in primary cilia can cause disorders such as deafness and left-right asymmetry reversals.

Microtubule Structure in Axonemes

• Each outer doublet consists of one complete microtubule (A tubule) and one incomplete microtubule (B tubule).

• A tubule has 13 protofilaments; B tubule has 10.

• Central pair microtubules are both complete.

Tubulin and Associated Proteins

• All microtubules contain tubulin and associated proteins.

• Tein is related to intermediate filaments and is a major component of axonemal dynein.

• Each doublet has a set of sidearms (dynein arms) that project from the A tubule.

Axonemal Dynein and Movement

Axonemal dynein is responsible for the sliding of microtubules against each other, which bends the axoneme and produces movement.

• Dynein arms occur in pairs (inner and outer).

• Adjacent doublets are linked by interdoublet links and radial spokes.

Radial Spokes and Doublet Sliding

• Radial spokes project inward toward the central pair and are important for transducing sliding into bending.

• During bending, adjacent outer doublets slide relative to one another, driven by dynein.

• This sliding is converted to localized bending, allowing cilia and flagella to beat.

Table: Comparison of Cilia and Flagella

Feature	Cilia	Flagella
Length	2–10 μm	Up to 200 μm
Number per cell	Many	Few (1–2)
Movement	Coordinated, parallel to cell surface	Whip-like, propels cell
Structure	9 + 2 axoneme	9 + 2 axoneme

Key Equations

• ATP hydrolysis by motor proteins: $\text{ATP} \rightarrow \text{ADP} + \text{P}_i$

• Rate of axonal transport: $\text{Rate} = 2\ \mu\text{m/sec}$

Example: Fast axonal transport in neurons is essential for moving neurotransmitter-containing vesicles from the cell body to the synapse, a process dependent on kinesin and dynein motor proteins.

Additional info: The notes have been expanded to include definitions, structural details, and functional context for all major topics, ensuring completeness and clarity for exam preparation.