Inverted Motility Assay: An In Vitro Technique to Visualize Myosin Movement on Immobilized Actin Filaments
JoVE Encyclopedia of Experiments
Biological Techniques
0 views • 3:39 min • July 8th, 2025
Myosin - a motor protein - has a globular head domain that interacts with actin - a filamentous protein. This interaction is essential for the translocation of myosin along the actin filaments. To study this binding in vitro, perform an inverted motility assay of myosin over surface-tethered actin.
To begin, assemble an assay chamber by placing a biotinylated polyethylene glycol - PEG-coated coverslip over a microscope slide such that the coated surface faces the inside, creating the top of the chamber.
Pass a solution containing avidin - a glycoprotein - through the chamber. Avidin binds to biotin with high affinity, which enhances the actin adhesion. Next, flow in biotin-tagged actin filaments labeled with a red fluorophore. The biotin ligand binds to avidin, immobilizing the filaments on the coverslip surface.
Finally, add green fluorescent protein-labeled myosin 5A proteins in a buffer containing adenosine triphosphate, or ATP, molecules. Dual-headed myosin 5A, containing a trailing and a leading head, interacts with the immobilized actin filaments. The trailing head of the myosin binds to an ATP and detaches from actin.
The hydrolysis of the ATP generates a power stroke, enabling the trailing head to bind to a position ahead of the leading head along the filament. Through successive power strokes, the myosin moves along the immobilized actin filament without detaching.
Using fluorescence microscopy, record the movement of the green myosin molecules on the red actin filaments.
Prepare the solutions for myosin 5a inverted motility assay, and keep them on ice.
Wash the chamber with 10 microliters of motility buffer with DTT. Flow in 10 microliters of 1 milligram per milliliter BSA in motility buffer with DTT. Repeat this twice, waiting a minute after the third wash. Use tissue or filter paper to wick the solution through the channel by gently placing the corner of the paper at the flow chamber exit.
Then, wash the chamber with 10 microliters of motility buffer with DTT three times. Flow in 10 microliters of the NeutrAvidin solution in motility buffer with DTT, and wait for one minute. Then, wash with 10 microliters of that solution three times.
Flow in 10 microliters of bRh-actin-containing motility buffer with DTT using a large-bored pipette tip, and wait for one minute. Then, wash with 10 microliters of motility buffer with DTT three times.
Flow in 30 microliters of final buffer with 10 nanomolar myosin 5a, and immediately load onto the total internal reflection fluorescence microscope. Begin recording once the optimum focus for TIRF imaging is found.
Overview
This article describes an inverted motility assay to study the interaction between myosin, a motor protein, and actin filaments. The assay allows for the observation of myosin movement along immobilized actin using fluorescence microscopy.
Key Study Components
Area of Science
- Neuroscience
- Cell Biology
- Biophysics
Background
- Myosin is a motor protein that interacts with actin filaments.
- This interaction is crucial for cellular movement and muscle contraction.
- Understanding this binding can provide insights into various biological processes.
- Fluorescence microscopy is used to visualize these interactions in real-time.
Purpose of Study
- To investigate the binding dynamics of myosin to actin filaments.
- To observe the movement of myosin along actin in a controlled environment.
- To enhance understanding of motor protein function at the molecular level.
Methods Used
- Preparation of an assay chamber with biotinylated PEG-coated coverslip.
- Flowing avidin and biotin-tagged actin filaments into the chamber.
- Adding green fluorescent protein-labeled myosin 5A proteins.
- Using total internal reflection fluorescence microscopy to record myosin movement.
Main Results
- Myosin 5A successfully binds to and moves along immobilized actin filaments.
- The movement is characterized by successive power strokes facilitated by ATP hydrolysis.
- Fluorescence microscopy effectively captures the dynamics of this interaction.
- The assay provides a reliable method for studying motor protein behavior.
Conclusions
- The inverted motility assay is a valuable tool for studying myosin-actin interactions.
- Findings contribute to the understanding of motor protein mechanics.
- This method can be applied to investigate other motor proteins and their functions.
Frequently Asked Questions
Executive Industry Relevance
This inverted motility assay enables direct visualization of myosin-actin motor protein interactions, providing mechanistic insights into force generation and translocation dynamics. By isolating the core actin-myosin interface in a controlled in vitro system, the assay supports target validation and de-risking of motor protein-focused therapeutic hypotheses. The quantitative readout of movement velocity and processivity offers predictive value for early discovery programs targeting cytoskeletal regulators.
Strategic Applications in Biopharma R&D
Early Discovery & Target Validation
- Scientific Value: Interrogates the functional competence of myosin motor domains through direct observation of actin-dependent translocation.
- Operational Value: Isolates myosin-actin binding from cellular complexity, enabling unambiguous attribution of observed effects to the target interaction.
- Scientific Value: Supports hypothesis testing regarding myosin isoform-specific adaptations and regulatory mechanisms.
Screening & Assay Development
- Scientific Value: Generates quantitative, real-time data on myosin motility parameters such as velocity, run length, and duty ratio.
- Operational Value: Establishes a standardized, reproducible platform for comparing wild-type and mutant myosin proteins under defined biochemical conditions.
- Scientific Value: Enables structure-function correlation by linking specific motor domain mutations to altered motility phenotypes.
Translational & Preclinical Research
- Scientific Value: Provides a disease-relevant system for evaluating pathogenic myosin variants associated with cardiomyopathies or neurological disorders.
- Operational Value: Facilitates preclinical assessment of small molecules or biologics designed to modulate motor protein activity.
- Scientific Value: Supports biomarker discovery by identifying motility signatures correlating with functional deficits in disease models.
Pipeline & Workflow Integration
The assay fits within the discovery continuum from target validation through lead optimization, offering a biophysical orthogonal readout to complement cellular and biochemical assays.
- Discovery Biology: Enables mechanistic de-risking by confirming target engagement and functional activity of myosin proteins outside cellular contexts.
- Screening: Delivers assay-ready, immobilized actin substrates compatible with high-resolution imaging and compound screening formats.
- Analytics: Outputs kinetic parameters (velocity, processivity, run length) that support comparative analysis across conditions and guide structure-activity relationships.
- Translational Research: Connects molecular motor dysfunction to phenotypic outcomes through controlled reconstitution of disease-associated variants.
- Enterprise Reuse: Represents a modular, adaptable platform applicable to multiple myosin isoforms and actin-binding proteins across therapeutic areas.
Operational & Enterprise Impact
- Scientific Value: Delivers direct, visual evidence of motor protein function, reducing reliance on indirect proxies and increasing confidence in target mechanism.
- Operational Value: Employs widely accessible fluorescence microscopy and standard protein labeling techniques, minimizing specialized equipment needs.
- Strategic Value: Improves go/no-go decisions by providing early, quantitative insight into motor protein behavior that predicts cellular and tissue-level outcomes.
- Portfolio Impact: Enables risk-adjusted prioritization of myosin-targeted programs based on biophysical validation of target function.
Implementation Considerations
- Requires expertise in protein purification, fluorescence labeling, and microfluidic flow chamber assembly.
- Depends on total internal reflection fluorescence (TIRF) microscopy for optimal signal-to-noise in single-molecule imaging.
- Necessitates standardization of actin preparation, myosin labeling, and buffer conditions to ensure reproducibility across users and sites.
- Involves optimization considerations when adapting to different myosin isoforms or actin variants with distinct biochemical properties.
- Practical limitations include photobleaching during extended imaging sessions and potential surface-induced artifacts affecting protein mobility.
Biopharma FAQs
Motility of Single Molecules and Clusters of Bi-Directional Kinesin-5 Cin8 Purified from S. cerevisiae Cells
Related Videos
0 Views
Probing Myosin Ensemble Mechanics in Actin Filament Bundles Using Optical Tweezers
Related Videos
0 Views
Using Microfluidics and Fluorescence Microscopy to Study the Assembly Dynamics of Single Actin Filaments and Bundles
Related Videos
0 Views
The Gliding Actin Filament Assay: An In Vitro Motility Assay to Study Translocation of Actin Filaments on Immobilized Myosins
Related Videos
0 Views
Myosin-Specific Adaptations of In vitro Fluorescence Microscopy-Based Motility Assays
Related Videos
0 Views
Dissecting Mechanoenzymatic Properties of Processive Myosins with Ultrafast Force-Clamp Spectroscopy
Related Videos
0 Views
Single-Molecule Analysis of Sf9 Purified Superprocessive Kinesin-3 Family Motors
Related Videos
0 Views
Reconstitution of Membrane-Tethered Minimal Actin Cortices on Supported Lipid Bilayers
Related Videos
0 Views
Visualizing Actin and Microtubule Coupling Dynamics In Vitro by Total Internal Reflection Fluorescence (TIRF) Microscopy
Related Videos
0 Views
The Mechanics of (Poro-)Elastic Contractile Actomyosin Networks As a Model System of the Cell Cytoskeleton
Related Videos
0 Views
Last updated: 22 August 2026