Point mutations in the genes encoding the skeletal muscle
isoforms of
tropomyosin can cause a range of muscle diseases. The amino acid substitution of Arg for Pro residue in the 90th position (R90P) in γ-
tropomyosin (Tpm3.12) is associated with
congenital fiber type disproportion and
muscle weakness. The molecular mechanisms underlying muscle dysfunction in this disease remain unclear. Here, we observed that this mutation causes an abnormally high Ca2+-sensitivity of myofilaments in vitro and in muscle fibers. To determine the critical conformational changes that
myosin, actin, and
tropomyosin undergo during the
ATPase cycle and the alterations in these changes caused by R90P replacement in Tpm3.12, we used polarized fluorimetry. It was shown that the R90P mutation inhibits the ability of
tropomyosin to shift towards the outer domains of actin, which is accompanied by the almost complete depression of
troponin's ability to switch actin monomers off and to reduce the amount of the
myosin heads weakly bound to
F-actin at a low Ca2+. These changes in the behavior of
tropomyosin and the
troponin-tropomyosin complex, as well as in the balance of strongly and weakly bound
myosin heads in the
ATPase cycle may underlie the occurrence of both abnormally high Ca2+-sensitivity and
muscle weakness. BDM, an inhibitor of
myosin ATPase activity, and
W7, a
troponin C antagonist, restore the ability of
tropomyosin for Ca2+-dependent movement and the ability of the
troponin-tropomyosin complex to switch actin monomers off, demonstrating a weakening of the damaging effect of the R90P mutation on muscle contractility.