Atp-induced Sliding of Microtubules in Bull Sperm Flagella

ATP-induced sliding of doublet microtubules has been recently demonstrated with axonemes of sea urchin sperm flagella which had been briefly digested with trypsin (13) . The specific chemical conditions in which such sliding occurs are the same as those required for the reactivation of beating in demembranated sea urchin sperm cells (4) . The conclusion from these observations, that the bending of sea urchin sperm flagella is based on a mechanism which produces local sliding between adjacent doublet microtubules, supports theoretical studies of flagellar motility which find that a "sliding filament" model is better than a "contracting filament" model in explaining some of the characteristics of the flagellar bending waves (2, 10) . The application of these results from sea urchin sperm flagella to explain the mechanism of mammalian sperm motility is complicated by the presence of large additional structures found in the flagella of mammalian sperm. Mammalian sperm flagella have in common with sea urchin sperm flagella a basic axoneme composed of a cylinder of nine parallel doublet microtubules surrounding a central pair of single microtubules (3, 1) . These 11 microtubules are held together by various accessory structures including pairs of arms which contain the ATPase protein dynein which seem to serve as mechanically active bridges between adjacent doublet tubules (5) . In mammalian sperm the basic axoneme is supplemented by nine coarse fibers which lie parallel and adjacent to the nine doublet microtubules forming a second cylindrical layer in the flagellar structure (3, 1) . The coarse fibers are much larger than the doublet tubules near the head of the sperm cell and taper gradually until they disappear near the tip of the flagellum . The microtubules and coarse fibers are enclosed by a third cylindrical layer consisting of a mitochondrial sheath near the sperm head and a fibrous sheath along the rest of the length of the flagellum . In order to produce motion in the more bulky mammalian sperm, it is generally assumed that the coarse fibers participate actively in the motile process although an inactive elastic role has also been suggested (7) . An inactive elastic role has also been proposed for the fibrous sheath (9) . In order to determine whether the doublet microtubules of mammalian sperm flagella are capable of motility similar to the doublets of sea urchin sperm flagella the effects of ATP on trypsindigested bull sperm were examined .