Locomotion and Movement · Lesson 3 of 10
Contractile Proteins: Actin, Myosin and Regulation
“Analyse the molecular composition of thin and thick filaments and explain how troponin, tropomyosin and ATP regulate cross-bridge binding.”
• Describe G-actin and F-actin in thin filaments. • Explain how tropomyosin and troponin regulate myosin-binding sites. • Identify heavy and light meromyosin regions. • Relate ATPase and actin-binding functions to the myosin head.
Thin and thick filaments have different jobs
Sarcomere contraction depends on two protein-based filaments arranged in parallel. The thin filament is built chiefly from actin and associated regulators; the thick filament contains many myosin molecules whose projecting heads can interact with actin. The filaments do not act by dissolving, folding into half their length or swapping places. Instead, interactions between many individual myosin heads and binding sites on actin create force that moves one filament relative to the other. Understanding their molecular structure explains how this repetitive motion can occur.
The actin filament is a helical polymer
A G-actin unit is a globular actin protein. Many G-actin monomers polymerise into a filamentous strand known as F-actin. Two such strands are twisted around one another in the textbook's model of a thin filament. The surface of actin contains sites that can bind myosin. Because these sites are repeated along the filament, successive myosin heads can engage different points as the thin filament moves. Actin is both a structural and an interactive component rather than a passive string.
Two strands of tropomyosin lie along the actin filament and are associated with a protein complex called troponin at intervals. In relaxed skeletal muscle, the troponin–tropomyosin system blocks or restricts access to the myosin-binding sites on actin. The central role of these regulatory proteins is to keep actin and myosin from strongly interacting until a calcium signal is present. Simply having ATP and both filament types inside a resting fibre does not cause sustained contraction when the sites remain blocked.
Calcium changes the regulatory arrangement
When intracellular Ca²⁺ rises following muscle activation, calcium binds to a regulatory subunit of troponin. This changes the arrangement of troponin and shifts tropomyosin relative to actin so that myosin-binding sites become accessible. The textbook states that calcium removes the masking of sites; the underlying mechanism is a protein conformational change, not chemical destruction of tropomyosin or actin. As Ca²⁺ is removed during relaxation, these regulatory proteins again reduce the accessibility of the interaction sites.
Myosin monomers form thick filaments
Each thick filament is an ordered assembly of many myosin molecules. The textbook uses the term meromyosin to describe a myosin structural unit with a globular head and short arm and a long tail. The head and short arm are referred to as heavy meromyosin (HMM), while the remaining tail region is called light meromyosin (LMM) in this simplified description. More precisely, HMM can contain both heads with adjoining portions of the molecular tails, and LMM denotes a tail-derived fragment in biochemical studies. The school model conveys the essential functional split: protruding motor heads and an assembling tail.
The globular head of myosin has a site for actin binding and a site for ATP interaction. It also behaves as an ATPase, meaning it can hydrolyse ATP to ADP and inorganic phosphate, releasing energy into the protein's conformational cycle. The tail helps organise multiple myosin molecules into a thick filament, while heads extend from its surface to form cross-bridges with adjacent thin filaments. Their orientation is arranged so that many heads can contribute to shortening in an organised direction.
ATP supplies both movement and detachment
One of the most important counterintuitive facts is that ATP is required to release myosin from actin after a strong-binding event. ATP binding to the head lowers its affinity for actin, causing detachment; subsequent hydrolysis prepares the head for another force-producing step. Therefore lack of ATP does not simply lead to completely relaxed muscle. Persistent cross-bridges can remain strongly attached if ATP is unavailable, contributing to stiffness such as rigor mortis after death. This application follows directly from the binding cycle.
Ca²⁺ and ATP therefore regulate different stages. Ca²⁺ makes actin sites available by acting on troponin. ATP and its hydrolysis regulate the myosin motor cycle, including detachment and preparation for a further stroke. The motor neuron signal raises calcium in the fibre, but the ATPase activity is located in myosin heads themselves. These two inputs allow the fibre to switch between contraction and relaxation while retaining the same filament proteins.
| Component | Structure | Role |
|---|---|---|
| G-actin | Globular subunit | Polymerises to F-actin |
| F-actin | Twisted filamentous strands | Thin-filament framework |
| Tropomyosin | Elongated regulatory strands | Masks/repositions myosin-binding sites |
| Troponin | Calcium-sensitive protein complex | Controls tropomyosin in response to Ca²⁺ |
| Myosin head | Globular HMM motor region | ATPase and actin binding |
| Myosin tail | LMM-associated coiled region | Assembles thick filament architecture |
Apply the molecular mechanism to a real sarcomere
Imagine actin binding sites blocked while a myosin head is ready to pull. Until calcium binds the regulatory complex, productive cross-bridges cannot form repeatedly. When Ca²⁺ increases, the sites become available and ATP-powered myosin heads bind and generate force. If troponin could not respond to calcium, contraction would fail despite normal filament structure. If myosin lacked ATPase activity, its energy-dependent conformational cycle would be disrupted. Structure and function thus connect directly to the sequence of muscle contraction.
Troponin senses Ca²⁺, tropomyosin covers/uncover actin's sites, actin supplies binding sites, and myosin heads perform ATP-dependent motor work.
Quick Quiz
Quiz
What is G-actin?
Which protein complex binds Ca²⁺ to help uncover actin sites?
What is the principal function of tropomyosin in relaxed muscle?
Which part of myosin has actin-binding and ATPase functions?
What happens when new ATP binds a strongly bound myosin head?
Practice Problems
- Explain the relationship among G-actin, F-actin and the thin filament.
- Describe how Ca²⁺ exposes the actin sites needed for contraction.
- Draw and label a myosin monomer using HMM, LMM, head, tail, ATP and actin sites.
- What is the role of ATP binding versus ATP hydrolysis in the myosin cycle?
- Why does calcium alone not provide all the energy of contraction?
Model Answers
1. G-actin monomers polymerise into F-actin strands; two strands form the thin-filament actin backbone, alongside tropomyosin and troponin regulatory complexes.
2. Ca²⁺ binds troponin, changing the troponin–tropomyosin arrangement and allowing myosin heads to access sites on actin, permitting cross-bridges.
3. Show a globular ATPase motor head with actin/ATP sites, a short projecting arm in the HMM region and a long tail corresponding to the LMM-associated fragment.
4. New ATP binding breaks strong actin–myosin interaction. Its hydrolysis prepares or cocks the myosin head to produce force in a subsequent cycle.
5. Ca²⁺ regulates actin-site availability, while chemical energy for repeated motor movements and detachment/re-cocking comes from ATP reactions at myosin heads.
Key Takeaways
• Thin filaments contain two F-actin polymers built from G-actin plus troponin and tropomyosin. • Ca²⁺ binding to troponin changes tropomyosin position and exposes actin binding sites. • Myosin molecules assemble into thick filaments, with motor heads extending outward. • Myosin heads contain actin-binding and ATP-binding sites and act as ATPase enzymes. • ATP binding supports cross-bridge detachment; ATP hydrolysis energises another motor cycle.