ABSTRACT Chara myosin is a plant myosin responsible for endoplasmic streaming in Chara corallina (australis), and classified into myosin class XI according to sequence homology in the head domain. Sequence analysis has revealed that it is a two-headed myosin with six IQ motifs in the neck, a long stalk and a globular tail. Despite the long stalk of coiled coil, it is soluble at low ionic strength owing to the large globular tail. Chara myosin is unique because of the fastest sliding movement observed so far: it slides F-actin at the maximum velocity of 60 μm/s, ten times of the speed of muscle myosin II. Our question is how Chara myosin slides so fast. Here, we extensively discuss if the lever arm model for muscle contraction mechanism, based on the idea of tight coupling, is relevant to understand the mechanism of the fast sliding of Chara myosin. We also discuss the mechanism from a different idea of loose coupling between the chemical reaction and the mechanical event.
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