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#flagella movement

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Sea urchin -l

Sea urchin sperm propulsion relies on:

Sea urchin sperm propulsion depends on highly conserved flagellar axoneme structure composed of nine outer microtubule doublets surrounding central pair with dynein arms and nexin links. Dynein heavy chain is ATPase motor protein that hydrolyzes ATP to slide adjacent doublet microtubules relative to each other, converting chemical energy into mechanical bending motion of flagellum that propels cell forward. Activation by intracellular pH rise stimulates dynein ATPase catalytic activity. Specific mutations or chemical inhibitors of dynein such as vanadate abolish motility while actin, myosin and tubulin alone provide structural tracks but no motor power. Thus dynein-driven microtubule sliding drives flagellar propulsion.

Ref: Gibbons, J Cell Biol 1981, Dynein in flagellar motility; Alberts, Molecular Biology of Cell, Chapter 20: Axoneme.

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Cytoskeleton

How do dynein arms generate bending in cilia and flagella?

Generation of ciliary and flagellar bending explained by sliding filament model originally proposed by Afzelius and refined by Satir Summers Gibbons and later cryo EM studies visualizing dynein power stroke in situ. Axonemal dynein heavy chains anchored on A tubule of doublet hydrolyze ATP in AAA ring inducing 8 nm movement bending of linker domain that swings stalk toward microtubule plus tip causing interdoublet sliding up to several hundred nanometers in disintegrated axonemes. Because doublets anchored at basal body microtubule organizing center and interconnected by elastic nexin links length 40 nm sliding cannot produce free translation but converts into local curvature generating bending moment. Successive coordinated activation on opposite sides of axoneme propagates bend wave from base to tip at 10 to 20 Hz frequency for cilia 50 Hz for flagella. ATP binding to AAA1 causes detachment low affinity state hydrolysis priming stroke Pi release power stroke inducing sliding between doublets. Thus dynein induces sliding between microtubule doublets by hydrolyzing ATP not depolymerizing tubulin at plus ends which would shorten axoneme contrary to constant length observation during beat cycle.

Ref: Satir et al., J Cell Biol 2014; Summers & Gibbons 1971 – Dynein arms generate bending by ATP hydrolysis sliding doublets.

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