The counter-bend phenomenon in dynein-disabled rat sperm
flagella and what it reveals about the inter-doublet
elasticity
Charles B. Lindemann 1*, Lisa J. Macauley 1 and Kathleen A. Lesich 1
1 Oakland University
* To whom correspondence should be addressed. E-mail: lindeman{at}oakland.edu.
Submitted on February 4, 2005
Revised on March 14, 2005
Accepted on 12 May 2005
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Abstract |
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Rat sperm that have been rendered passive by disabling the dynein motors with 50 µM sodium metavanadate and 0.1 mM ATP exhibit an interesting response to imposed bending. When the proximal flagellum is bent with a microprobe, the portion of the flagellum distal to the probe contact point develops a bend in the direction opposite the imposed bend. This "counter-bend" is not compatible with a simple elastic beam. It can be satisfactorily explained by the sliding tubule model of flagellar structure, but only if there are permanent elastic connections between the outer doublets of the axoneme. The elastic component that contributes the bending torque for the counter-bend does not reset to a new equilibrium position following an imposed bend, but returns the flagellum to a nearly straight or slightly curved final position after release from the probe. This suggests it is based on fixed, rather than mobile, attachments. It is also disrupted by elastase or trypsin digestion, confirming that it is dependent on a protein linkage. Adopting the assumption that the elasticity is attributed to the nexin links that repeat at 96 nm intervals, we find an apparent elasticity for each link that ranges from 1.6 to 10 x 10-5 N/m. However, the elasticity is non-linear and does not follow Hooke's law, but appears to decrease with increased stretch. In addition, the responsible elastic elements must be able to stretch to more than ten times their resting length without breakage to account for the observed counter-bend formation. Elasticity created by some type of protein unfolding may be the only viable explanation consistent with both the extreme capacity for extension and the nonlinear character of the restoring force that is observed.
Key Words:
Geometric Clutch, axoneme, flagellar stiffness, nexin links, outer doublets, radial spokes
