Arrangement of tubulin subunits and microtubule-associated proteins in the central-pair microtubule apparatus of squid (Loligo pealei) sperm flagella.

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Abstract
A comprehensive high-resolution structural analysis of the central-pair microtubule apparatus of sperm flagella is given. It describes the arrangement of several microtubule-associated sheath components and suggests that microtubules are structurally asymmetric. The 2 microtubules of the central pair differ in several respects: the C2 tubule bears a single row of 18-nm-long sheath projections with an axial periodicity of 16 nm, the C1 tubule possesses rows of 9-nm globular sheath components with an axial repeat of 32 nm. The lumen of the C2 tubule always appears completely filled with electron-dense material; that of the C1 tubule is frequently hollow. The C2 tubule possesses a series of beaded chains arranged around the microtubule; the beaded chains are composed of globular subunits 7.5-10 nm in diameter and appear to function in the pairing of the C1 and C2 tubules. These findings indicate that the beaded chains are not helical, but assume the form of lock washers arranged with a 16-nm axial periodicity on the microtubule, and that the lattice of tubulin dimers in the C2 tubule is not helically symmetric, but that there are seams between certain pairs of protofilaments. Proposed lattice models predict that, because of these seams, central pair and perhaps all singlet microtubules may contain a ribbon of 2-5 protofilaments that are resistant to solubilization.