The Magnitude of the Transverse-Force in Flagella at the Point of Beat Reversal.

Charles B. Lindemann

The transverse-force (t-force) acting across the flagellar axoneme was estimated by two independent methods. The first method used videotaped data of reactivated Triton X-100 extracted bull sperm that had had their flagella shortened by micro-dissection. When the flagellum is shortened to 15-20 microns the beat sporadically arrests at the extremes of the beat stroke. The curvature of the flagellum at the arrest point and the stiffness of the flagellum were measured and used to find the bending torque and the linear tension on the doublets. The curvature multiplied by the linear tension gave t-force values in the range of 0.38 to 0.5nN/micron. A second method utilized micrographs of beating flagella of bull sperm. The maximum curvature of newly formed bends was measured. The distance between the peak of the bend and the approximate point on the preceding bend where curvature starts to decrease was measured to determine the length of the region of active dyneins, and from this length the number of participating dyneins was found. The isometric dynein-force in bull sperm is ~5pN/head (Schmitz et al. 2000, Biophys. J. 79:468-478). The average sliding velocity is 4 microns/s in swimming bull sperm at 23C. Accounting for force-velocity behavior of dynein, the average force should be ~1.2pN/head. The tension on the doublets was found from the force/head and the number of contributing dynein heads. The tension multiplied by the measured curvature yielded a t-force at the switch-point of approximately 0.5nN/micron for swimming bull sperm, in agreement with the value from the arrested bull sperm. This magnitude of t-force is roughly equal to the total force that can be generated by all the dyneins on one micron of outer doublet. Therefore, the t-force may be capable of overpowering the dyneins to switch them off. Supported by NSF grant MCB-0110024.