Discovery of ultrafast myosin, its amino acid sequence, and structural features.

Cytoplasmic streaming with extremely high velocity (∼70 μm s) occurs in cells of the characean algae (). Because cytoplasmic streaming is caused by myosin XI, it has been suggested that a myosin XI with a velocity of 70 μm s, the fastest myosin measured so far, exists in cells. However, the velocity of the previously cloned myosin XI (XI) was about 20 μm s, one-third of the cytoplasmic streaming velocity in Recently, the genome sequence of has been published, revealing that this alga has four myosin XI genes. We cloned these four myosin XI (XI-1, 2, 3, and 4) and measured their velocities. While the velocities of XI-3 and XI-4 motor domains (MDs) were similar to that of XI MD, the velocities of XI-1 and XI-2 MDs were 3.2 times and 2.8 times faster than that of XI MD, respectively. The velocity of chimeric XI-1, a functional, full-length XI-1 construct, was 60 μm s These results suggest that XI-1 and XI-2 would be the main contributors to cytoplasmic streaming in cells and show that these myosins are ultrafast myosins with a velocity 10 times faster than fast skeletal muscle myosins in animals. We also report an atomic structure (2.8-Å resolution) of myosin XI using X-ray crystallography. Based on this crystal structure and the recently published cryo-electron microscopy structure of acto-myosin XI at low resolution (4.3-Å), it appears that the actin-binding region contributes to the fast movement of myosin XI. Mutation experiments of actin-binding surface loops support this hypothesis.