Evidence against essential roles for subdomain 1 of actin in actomyosin sliding movements

We have engineered acto-S1chimera proteins carrying the entire actin inserted in loop 2 of the motor domain of Dictyostelium myosin II with 24 or 18 residue-linkers (CP24 and CP18, respectively). These proteins were capable of self-polymerization as well as copolymerization with skeletal actin and exhibited rigor-like structures. The MgATPase rate of CP24-skeletal actin copolymer was 1.06 s−1, which is slightly less than the Vmax of Dictyostelium S1. Homopolymer filaments of skeletal actin, CP24, and CP18 moved at 4.7 ± 0.6, 2.9 ± 0.6, and 4.1 ± 0.8 μm/s (mean ± SD), respectively, on coverslips coated with skeletal myosin at 27 °C. Statistically thermodynamic considerations suggest that the S1 portion of chimera protein mostly resides on subdomain 1 (SD-1) of the actin portion even in the presence of ATP. This and the fact that filaments of CP18 with shorter linkers moved faster than CP24 filaments suggest that SD-1 might not be as essential as conventionally presumed for actomyosin sliding interactions.