Strikingly different propulsive forces generated by different dynein‐deficient mutants in viscous media

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Abstract
The propulsive force generated by Chlamydomonas mutants deficient in flagellar dynein was estimated from their swimming velocities in viscous media. The force produced by wild‐type cell increased by 30–40% when viscosity was raised from 0.9 to 2 cP but decreased as viscosity was further raised above 6 cP. The biphasic dependence of force generation on viscosity was also observed in the mutant idal, which lacks the II component of the inner‐arm dynein. The mutant ida4, which lacks the inner‐arm 12 component, was extremely susceptible to viscosity and stopped swimming at 6 cP, at which other mutants could swim. In contrast, odal, which lacks the entire dynein outer arm, produced a fairly constant force of about one‐third of the wild‐type value, over a viscosity range of 0.9–11 cP. In demembranated and reactivated cell models of the wild type, the propulsive force decreased monotonically as viscosity increased. Thus the increase in force generation at about 2 cP observed in live cells may be caused by some unknown mechanism that is lost in cell models. The cell models of odal, in contrast, did not show a marked change in force generation with the change in viscosity. These results indicate that the force generation by the outer‐arm dynein greatly depends on viscosity or the velocity of movement, whereas the complete set of inner‐arm dynein present in the odal axoneme produces a fairly constant force at different viscosities. These different properties of inner and outer dynein arms should be important in the mechanism that produces flagellar beating.