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The myosin II SH1 helix is a joint that links the converter subdomain to the rest of the myosin motor domain and possibly plays a key role in the arrangement of the converter/lever arm. Several point mutations within the SH1 helix in human myosin IIs have been shown to cause diseases. To reveal whether these SH1 helix mutations affect not only motile activities but also thermal properties of myosin II, here we introduced the E683K or R686C point mutation into the SH1 helix in Dictyostelium myosin II. Thermal inactivation as well as thermal aggregation rates of these mutant proteins demonstrated that these mutations decreased the thermal stability of myosin II. Temperature dependence of sliding velocities of actin filaments showed that these mutations also reduced the activation energy of a rate-limiting process involved in actin movement. Given that these mutations are likely to alter coupling between the subdomains, and thus their thermal fluctuations, we propose that the SH1 helix is a key structural element that determines the flexibility and thermal properties of the myosin motor. These characteristics of the SH1 helix may contribute to the pathogenesis of the human diseases caused by mutations within this structural element.

A human myopathy has been reported to be caused by a point mutation in a fast myosin heavy chain gene. This myopathy is autosomal dominant inclusion-body myopathy type3 and is caused by the mutation, which change glutamic acid into lysine at the 706th amino acid, is located at the SH1 helix region of the myosin II motor domain. The mutation in the SH1 helix is located a highly conserved region of motor domain. The SH1 helix region acts as a linker for transmitting the structural changes of ATP-binding site in the catalysis domain to the lever arm. To investigate the effect of the mutation on the actin-myosin motility, we have introduced a corresponding mutation into the SH1 helix of Dictyosterium myosin II (E683K). The mutation resulted in a decrease in the actin-myosin sliding velocity (about 1/3 of the wild type), a decrease in thermal stability and the thermal aggregation of the myosin, which might be implicated in the disease process. This suggests that SH1 helix has an important function in the motor activity and the ability to interact with F-actin is reduced.