Myosin's chaperone, Unc45b (green), relocates from the Z-line (red) to the A-band when muscle cells are stressed.

Like overprotective parents, muscle myosin's chaperones don't stray far from their charge, according to images from Etard et al.

Muscle contraction depends on the proper arrangement of the myosin motor within the basic repeating muscle subunit, known as the sarcomere. The steps involved in constructing the sarcomere are not well-understood. As yet, an in vitro assembly system has not been devised, so researchers rely on the zebrafish system to unravel vertebrate muscle assembly.

As zebrafish muscles develop, their myosin is folded into a part of the sarcomere known as the A-band. Folding occurs with the help of two chaperones, called Unc45b and Hsp90a. Evidence in worm suggests that these chaperones remain with myosin in the A-band even after the muscle is fully formed. But Etard et al. found that their association with the A-band in zebrafish was much more short-lived: only by slowing down muscle development could the authors catch a glimpse of the chaperones in the forming A-band.

In normally developing fish, the chaperones made a swift relocation to a nearby sarcomere structure called the Z-line, where the large protein titin anchors myosin to actin filaments. This location was not permanent, however. The chaperones returned to the A-band, the group found, when muscles were stressed. Stresses that mobilized the chaperones included heat and cold shocks and damage to the muscle cell membrane.

The group proposes that these sorts of challenges somehow lead to myosin unfolding. In its unfolded form, they suggest, myosin's affinity for the chaperones is high enough to draw them away from their Z-line residence. FRAP measurements revealed that the two chaperones were highly dynamic at the Z-line, constantly shuttling between the sarcomere and the cytoplasm. Given the close proximity of the Z-line and the A-band, this dynamic behavior would allow the chaperones to probe myosin's condition repeatedly and help correct any folding errors.

Etard, C., et al.
J. Cell Biol.