ADP increases the switching threshold of the bull sperm beat, an effect that can be explained by increased microtubule binding affinity of dynein.
Kathleen A. Lesich, Dominic W. Pelle and Charles B. Lindemann, Dept of Biology, Oakland University, Rochester, MI 48309.
Triton X-100 extracted bull sperm reactivated with 0.1mM Mg-ATP show reduced beat frequency and increased beat amplitude in the presence of 1 to 4 mM ADP. We found no change in the isometric force produced when sperm flagella were stalled against a calibrated glass microprobe with 1mM ADP present. Since sperm flagella bend to a higher curvature after ADP treatment, we explored the possibility that ADP treated sperm flagella are more flexible. We measured the passive stiffness of 50 ÁM sodium vanadate treated bull sperm in the presence and absence of 4mM ADP and found no difference. The bending torque acting on a flagellum can be found by multiplying the flagellar stiffness by the curvature at any time that the viscous drag is negligible. ADP (1 and 4 mM) produced a significant increase in the bending torque in beating sperm flagella at the point of beat reversal were drag is nearly zero. ADP also significantly increased the bending torque in flagella that were shortened by dissection to a length where the beating showed transient arrests, and drag is zero. Our torque estimates allowed us to calculate the t-force acting in the flagella at the point of beat reversal. The t-force at the switch-point of the beat is increased significantly by ADP, averaging 0.6 nN / Ám with 0.1 mM ATP alone, and increasing to 2.8 and 1.6 nN / Ám with 1 mM and 4 mM ADP, respectively, added . Interpreted in terms of the Geometric Clutch hypothesis, this result can be explained if ADP is exerting its effect by increasing the adhesion of dynein, which is the tenacity of dynein attachment to the B-subtubule. This result is consistent with a regulatory effect of ADP on the microtubule binding affinity of dynein. Supported by N.S.F.grant MCB 0516181.