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Sperm flagella derive their motive power from the motor protein dynein. In this study, we show that maintenance of the flagellar beat cycle in detergent-extracted bull sperm models is highly dependent on the ratio of Mg2+ to adenosine triphosphate (ATP). An excess of either ATP un-complexed with Mg2+, or an excess of Mg2+ without an equivalent concentration of ATP, results in the loss of beat amplitude and a reduced curvature development in the beat cycle. In addition, we find that adenosine diphosphate (ADP) can stabilize the beat cycle and permit rhythmic beating across a broader range of ATP and Mg2+ concentrations. We provide evidence that suggests that when ATP is un-complexed with Mg2+, it disrupts the beat cycle by reducing dynein adhesion and thereby, reduces the transmission of dynein-generated force between the doublets. Excess Mg2+ does not act by the same mechanism and induces a condition where the flagellum is more resistant to bending. This is consistent with the idea that high Mg2+ stabilizes rigor bridges, and ATP reduces the microtubule binding affinity of dynein. Our results may explain how intact sperm are able to sustain coordinated flagellar beating under a wide range of metabolic conditions, as intact sperm produce ADP in direct proportion to their consumption of ATP. © 2014 Wiley Periodicals, Inc. |
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