Structural studies of stationary principal bends and of definitive patterns of spontaneous microtubule sliding disruption permitted description of the bending axis in sea urchin sperm tail axonemes. Lytechinus pictus sperm were demembranated in a buffer containing Triton X-100 and EGTA. Subsequent resuspension in a reactivation buffer containing 0.4 mM CaCl2 and 1.0 mM MgATP2- resulted in quiescent, rather than motile, cells and each sperm tail axoneme took on an extreme, basal principal bend of 5.2 rad. Thereafter, such flagellar axonemes began to disrupt spontaneously into two subsets of microtubules by active sliding requiring ATP. Darkfield light microscopy demonstrated that subset "1" is composed of microtubules from the inside edge of the principal bend. Subset "2" is composed of microtubules from the outside edge of the principal bend and always scatters less light in darkfield than subset 1. Subset 2, which always slides in the proximal direction, relative to subset 1, results in a basal loop of microtubules, and the subset 2 loop is restricted to the bend plane during sliding disruption. Electron microscopy revealed that doublets 8, 9, 1, 2, 3 and the central pair comprise subset 1, and doublets 4, 5, the bridge, 6, and 7 comprise subset 2. The microtubules of isolated subset 2 are maintained in a circular arc in the absence of spoke-central pair interaction. Longitudinal sections show that the bending plane bisects the central pair. We therefore conclude that the bend plane passes through doublet 1 and the 5-6 bridge and that doublet 1 is at the inside edge of the principal bend. Experimental definition of the axis permits explicit discussion of the location of active axonemal components which result in Ca2+-induced stationary basal bends and explicit description of components responsible for alternating basal principal and reverse bends.

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