Protein switches in muscle contraction.

Two types of Ca2+-sensitive protein complexes control the contraction of muscle: Troponin (TN) and tropomyosin (TM) are associated with the thin actin filaments, and a specific light chain is a regulatory subunit of myosin itself. Most muscles have both types of regulation. X-ray diffraction diagrams from whole muscle have shown changes in the position of tropomyosin and changes in the pattern of myosin crossbridge attachment associated with different states of the regulatory switches. The full interpretation of these diagrams is often ambiguous, however, and structural studies of the purified proteins provide essential information. Recent crystallographic results reveal that the TM molecule has unusual local domains of marginal stability, leading to extensive motions of the tropomyosin filaments. Electron microscopy of negatively stained thin filaments decorated with subfragments of scallop myosin yields unusually detailed images that show marked conformational changes in myosin crossbridges that are dependent on the presence or absence of the regulatory light chain. These observations suggest that both the special dynamic design of tropomyosin and the striking structural changes in the myosin crossbridges are significant clues for detailed models for the regulatory mechanism.