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

This study provides a comprehensive, high-resolution structural analysis of the central-pair microtubule apparatus of sperm flagella. It describes the arrangement of several microtubule-associated 'sheath' components and suggests, contrary to previous thinking, that microtubules are structurally asymmetric. The two microtubules of the central pair are different in several respects; the C₂ tubule bears a single row of 18-nm-long sheath projections with an axial periodicity of 16 nm, whereas 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 also 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; these models are supported by the results of the accompanying paper.