Two key players in muscle contraction at the sub-cellular level are actin and myosin, found on the thin and thick filaments, respectively, and are significant parts of a sarcomere’s structure (the contractile unit of muscle). Myosin has two heads, each similar in shape to the ends of golf clubs. During contraction, myosin heads attach and pull on actin strands, sliding the thin filament towards the center of the sarcomere, making it shorter. With lots and lots (and lots) of sarcomeres lined up in series in muscle, these tiny shortening events add up to a visible and functional contraction of the muscle.
When not interacting with actin, myosin exists in equilibrium between two biochemical states: A disordered relaxed state (DRX) and a structurally defined, ordered state termed the super-relaxed state (SRX) of myosin. The SRX state has been defined biochemically as having lower energy usage (ATPase activity). It is thought that the biochemically-defined SRX state corresponds to the structurally-defined interacting heads motif (IHM) state of myosin, though paradoxical data exists in the literature that doesn’t align with this hypothesis.
Recent research from an international collaboration between the Regnier and Moussavi-Harami Labs at the UW, and Michael Geeves, Emertius Professor of Physical Biochemistry, University of Kent, England, brings some clarity and a word of caution to the study of the potential states of relaxed myosin.
Lead authors Dr. Saffie Mohran (above, left), Dr. Kristi Kooiker (above, middle), and Max Mahoney-Schaefer (above, right) show through careful experimentation that a biochemical assay, which has been previously used to define the ratio of the SRX to DRX state of myosin, does not reliably quantify this ratio. They suggest a resolution for the paradoxical data in the field and emphasize the need for researchers to carefully determine the type of information that their assay provides and does not provide.
“The results of this study serve, in part, as a cautionary tale that the proper control measurements for processes on different time scales and magnitudes need to be performed and reported to validate the results and conclusions of the study,” said senior author Dr. Regnier. “In this case, the properties of the fluorescent probe mant-ATP, used to determine ATPase rates, are sensitive to long-term exposure and concentration relative to the protein enzyme (myosin) being studied.”
The findings not only contribute to our understanding of muscle biochemistry but also highlight the intricacies involved in deciphering the mechanisms underlying muscle contraction and energy conservation. As research in this field progresses, continued scrutiny and refinement of experimental techniques will be essential to reliably unraveling the complexities of muscle function.
Read the full article in the January Issue of the Journal of Biological Chemistry:
Mohran S, Kooiker K, Mahoney-Schaefer M, Mandrycky C, Kao K, Tu AY, Freeman J, Moussavi-Harami F, Geeves M, Regnier M. The biochemically defined super relaxed state of myosin-A paradox. J Biol Chem. 2024 Jan;300(1):105565. doi: 10.1016/j.jbc.2023.105565. Epub 2023 Dec 14. PubMed PMID: 38103642; PubMed Central PMCID: PMC10819765.