Myosin Filaments of Vertebrate Skeletal and Cardiac Muscle are Highly Similar, but not Identical

This study demonstrates that the structure of relaxed rabbit skeletal muscle thick filaments is highly similar to that of human and mouse cardiac muscle, differing mainly in the positioning of myosin binding protein C, which highlights distinct evolutionary solutions for muscle control and endothermy in mammals compared to the highly variable structures of insect flight muscle.

The Gist

The Big Picture: Muscle's "Engine Room"

Imagine your body is a massive city. The muscles are the power plants that keep the city running. Inside every muscle cell, there are tiny, microscopic engines called thick filaments. These engines are made mostly of a protein called myosin, which acts like a rowing crew pulling on ropes (actin filaments) to create movement.

For a long time, scientists thought these engines looked very different depending on whether they were in your heart (which beats constantly) or your arm (which you use to lift things). They also knew that insects had very different engines for their wings.

The main discovery of this paper is a surprise: The engines in a rabbit's leg muscle (skeletal) are almost identical to the engines in a human or mouse's heart (cardiac). They are built on the exact same blueprint, with only tiny, specific tweaks.

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The Analogy: The "Three-Lane Highway"

To understand how these muscles work, imagine the thick filament as a three-lane highway running down the center of the muscle fiber. The "cars" on this highway are the myosin heads (the parts that actually pull).

The researchers found that these cars are parked in three distinct lanes, each with a different job:

1. The "Deep Storage" Lane (IHM-S): These cars are parked deep inside the highway, tucked away in a secure garage. They are the most stable and are used for long-term energy conservation. They are the "reserve" troops.
2. The "Modulated Reserve" Lane (IHM-C): These cars are in the middle lane. They are ready to go but are being watched closely by a traffic controller.
3. The "Ready-to-Go" Lane (IHM-D): These cars are on the outermost lane, closest to the exit ramp. They are loosely packed and ready to jump into action immediately when a signal comes.

The Surprise: This "three-lane" system exists in both the rabbit's leg and the human heart. Even though the heart needs to beat steadily and the leg needs to sprint, they use the exact same parking structure. It's like finding that a Formula 1 race car and a family minivan use the exact same engine block, just tuned slightly differently.

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The "Traffic Controller" and the "Molecular Ruler"

Two other proteins play huge roles in this system:

• Titin (The Molecular Ruler): Imagine a giant, stretchy measuring tape running down the center of the highway. It holds everything in place and ensures the "cars" are spaced perfectly. The paper found that this ruler looks exactly the same in rabbits and humans. It's the universal blueprint for muscle construction.
• MyBP-C (The Traffic Controller): This is a protein that sits on the side of the highway, telling the myosin cars when to start pulling and when to rest.
  • The Difference: In the heart, this controller is a bit more "compact" and tucked in. In the rabbit's leg, the controller has a slightly different shape (specifically, a missing loop of protein) that allows it to grab onto the "Molecular Ruler" (Titin) in a different spot.
  • Why it matters: This tiny difference allows the leg muscle to react faster and more explosively, while the heart muscle stays steady and rhythmic. It's like the difference between a traffic light that turns green instantly for a race car versus one that cycles slowly for city traffic.

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The "Magic Drug" (Mavacamten)

To see these tiny structures clearly, the scientists used a drug called Mavacamten.

• What it does: Think of Mavacamten as a "parking brake" for the myosin engines. It forces the myosin heads to fold up and stay still (in the "Interacting Heads Motif").
• Why they used it: Normally, these engines are moving too fast to photograph. By hitting the "parking brake," the scientists could freeze the engines in a relaxed state and take a high-resolution 3D photo (using a super-powerful microscope called Cryo-EM).
• The Twist: This drug was originally designed for human heart disease. The scientists used it on rabbit leg muscles and found it worked there too, proving that the "parking mechanism" is the same in both types of muscle.

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Why Does This Matter? (The Evolutionary Story)

The paper ends with a fascinating thought about evolution:

• Insects: Insect flight muscles are like specialized, high-speed race tracks. They are built for one specific thing: flapping wings at a super-high frequency. They are very different from each other and very different from us.
• Vertebrates (Us): We are warm-blooded (endothermic). We need to generate heat to stay warm, and we need muscles that can do everything from holding a pose to sprinting.
• The Solution: Evolution didn't invent a new engine for every new job. Instead, it took one incredibly versatile, high-quality engine design (the one found in the rabbit leg and human heart) and kept it. It's a "universal engine" that can be tuned for different tasks.

In short: Nature found a perfect design for a muscle engine about 80 million years ago. Whether you are a mouse, a human, or a rabbit, your muscles are built on this same, highly adaptable blueprint. The only difference is how the "traffic controller" (MyBP-C) is parked, which allows your heart to beat steadily while your legs can run fast.

Technical Summary

1. Problem Statement

Vertebrate striated muscles (skeletal and cardiac) rely on the interaction between actin (thin) and myosin (thick) filaments for contraction. While the structure of actin filaments is highly conserved, the detailed architecture of myosin thick filaments has remained less understood, particularly in fast skeletal muscle.

• Knowledge Gap: High-resolution structures of vertebrate cardiac thick filaments have recently been elucidated using cryo-electron microscopy (cryoEM), revealing complex arrangements of myosin heads, tails, and accessory proteins (Titin, Myosin-Binding Protein C or MyBP-C). However, a comparable high-resolution structure for vertebrate skeletal muscle had not been reported.
• Scientific Question: Are the thick filaments of fast skeletal muscle structurally identical to those of cardiac muscle, or do they possess distinct architectural features that reflect their different physiological roles (e.g., recruitment-based force control in skeletal muscle vs. beat-to-beat modulation in cardiac muscle)?

2. Methodology

The authors employed single-particle cryo-electron microscopy (cryoEM) to reconstruct the 3D structure of native, frozen-hydrated thick filaments from rabbit psoas muscle (a fast-twitch skeletal muscle model).

• Sample Preparation:
  • Glycerinated rabbit psoas muscle was treated with elastase to release thick filaments.
  • Sarcomeric actin was removed using gelsolin to isolate pure thick filaments.
  • Mavacamten, a small-molecule drug known to stabilize the "interacting heads motif" (IHM) in cardiac myosin, was added at 150 µM. This was crucial to lock the myosin heads in a relaxed, super-relaxed state, facilitating high-resolution reconstruction.
• Data Acquisition:
  • Data was collected on a 300 kV Titan Krios microscope with a Falcon IV direct electron detector.
  • Approximately 17,280 high-quality movies containing well-dispersed filaments were retained from a total of 56,189.
• Image Processing:
  • Processing was performed using cryoSPARC v4.6.2.
  • Filament tracing and automated picking were used to extract segments.
  • After 2D classification and ab-initio 3D reconstruction, heterogeneous refinement yielded a map with a global resolution of ~10 Å.
  • The reconstruction focused on the C-zone (the central region of the thick filament where MyBP-C is located), which has a 430 Å axial repeat.
• Comparative Analysis:
  • The skeletal structure was compared against existing high-resolution maps of human and mouse cardiac thick filaments (e.g., EMD-15353, EMD-29722).
  • Atomic models of cardiac proteins were docked into the skeletal density maps to assess conservation.

3. Key Contributions

• First High-Resolution Skeletal Structure: This study provides the first subnanometer-resolution (10 Å) 3D reconstruction of isolated vertebrate skeletal muscle thick filaments.
• Validation of Conservation: It confirms that the fundamental architecture of the vertebrate thick filament is highly conserved between skeletal and cardiac muscle, despite their different physiological functions.
• Identification of Specific Divergence: The study pinpointed a specific structural difference in the interaction between fast MyBP-C (fMyBP-C) and Titin, which distinguishes skeletal from cardiac filaments.

4. Key Results

A. Conserved Myosin Head and Tail Architecture

• Interacting Heads Motif (IHM): The skeletal filament displays three distinct types of myosin head crowns (IHM-D, IHM-C, and IHM-S) arranged in triplets, identical to the cardiac filament.
  • IHM-S (Tilted): Forms the inner core, tightly packed, analogous to the "tilted crowns" (CrT) in cardiac muscle.
  • IHM-C (Horizontal): Forms an intermediate layer, analogous to "horizontal crowns" (CrH).
  • IHM-D (Disordered): Forms the outermost sheath; these heads are structurally disordered and likely the first to activate.
• Myosin Tails: The arrangement of myosin tails into three concentric layers (S, C, D) is nearly identical to cardiac filaments. The IHM-S tails form a solid, load-bearing core, while IHM-D tails form a loose outer sheath.
• Titin Backbone: The paired Titin strands (Titin-C and Titin-M) form a "molecular ruler" with an 11-domain super-repeat (T1–T11) that dictates the 430 Å periodicity. The conformation of Titin in skeletal muscle is virtually identical to that in human cardiac muscle.

B. The Critical Difference: fMyBP-C and Titin Interaction

While the overall scaffold is conserved, the positioning of fMyBP-C (fast skeletal MyBP-C) differs from cardiac MyBP-C (cMyBP-C):

• C-Terminal Domains (C8–C10): In both muscle types, these domains bind near the myosin backbone. However, in skeletal muscle, they are separated from Titin by the IHM-D tail layer.
• N-Terminal/Middle Domains (C5–C7):
  • Cardiac: The C5 domain is often "tucked in" near the myosin regulatory light chain, stabilized by a cardiac-specific 28-residue insert in the C5 loop.
  • Skeletal: The fMyBP-C C5 domain extends away from the myosin neck and makes a direct contact with Titin domain T8.
• Mechanism: The absence of the destabilizing cardiac-specific loop in fMyBP-C allows the C4-C5 region to adopt a more open conformation, enabling it to pivot and bind Titin T8. This creates a unique mechanical linkage between the Titin filament and the MyBP-C collar in skeletal muscle that is absent in cardiac muscle.

C. Mavacamten Binding

The study successfully utilized mavacamten (typically a cardiac drug) to stabilize the skeletal IHM, though it required a higher concentration (150 µM) compared to cardiac studies, suggesting slight differences in the binding pocket affinity between isoforms.

5. Significance and Implications

• Evolutionary Insight: The results suggest that the vertebrate thick filament architecture is a highly adaptable, conserved "blueprint" that has remained stable for over 80 million years (since the divergence of rabbits, mice, and humans). This contrasts sharply with invertebrate flight muscles, which show high structural heterogeneity.
• Mechanistic Understanding of Muscle Regulation:
  • The trilayered structure (IHM-S, C, D) provides a physical basis for the "hierarchy of control," allowing graded recruitment of myosin heads.
  • The fMyBP-C–Titin interaction in skeletal muscle suggests a direct mechanical coupling between the elastic Titin spring and the regulatory MyBP-C collar. This may allow skeletal muscle to sense tension and modulate contraction speed and energy cost differently than cardiac muscle.
• Thermogenesis vs. Efficiency: The paper posits that the conserved, adaptable vertebrate thick filament supports endothermy (heat generation) in mammals by allowing variable levels of myosin activation and sequestration. In contrast, insect flight muscles are specialized for narrow, high-frequency contraction with different structural solutions.
• Clinical Relevance: Understanding the subtle structural differences between skeletal and cardiac MyBP-C is vital for developing targeted therapies for muscle diseases (e.g., distal arthrogryposis linked to fMyBP-C mutations) without inadvertently affecting cardiac function.

In conclusion, this paper establishes that while vertebrate skeletal and cardiac thick filaments share a nearly identical structural core, they diverge in the specific regulatory interactions of MyBP-C with Titin, reflecting their distinct physiological demands for force generation and energy regulation.