Microtubules: Evidence for 13 Protofilaments

Abstract

When microtubules are fixed in glutaraldehyde in the presence of tannic acid and thin sections cut, the subunit structure of the microtubule is readily observed without the need of image reinforcement. Seven types of microtubules were analyzed: those in the heliozoan axoneme, the mitotic apparatus, the contractile axostyle, repolymerized microtubules derived from the chick brain, the central pair in flagella, and the A tubules of flagella and the basal body. In all cases microtubules were composed of 13 equally spaced protofilaments. The B tubules in flagella and the basal body appear to be composed of 11 subunits. The connections of the B to the A and the C to the B are described. A model of a microtubule is presented.

Functional protofilament numbering of ciliary, flagellar, and centriolar microtubules

“…The cryo-EM tomography of Sui and Downing [2006] on sea urchin flagella also resolved 10 B-PFs, but differs from other reports in that it suggests a continuous gap between the A-and B-tubules with no component in the gap; instead, a linker was described running from PFs A 12-13 across the B-lumen to PFs B 9-10 (converting their PF numbering to the convention presented here- Fig. 1) The numbering pattern of Tilney et al [1973] followed the convention of Witman et al [1972b] (Fig. 2a) to number the B-tubule CCW beginning with tubulin PF B 1 , which forms the outer junction with the A-tubule, leading to tubulin PF B 10 , and ultimately to the B-11th component, morphologically distinguishable from a tubulin PF.…”

“…In addition to being consistent with the B-tubule PF numbering system of Witman et al [1972b] and Tilney et al [1973], there are several reasons for also numbering the A-tubule PFs CCW [Linck, 1976], beginning with PF A 1 - Fig. 1c.…”

“…Even though the B-11th component will eventually be identified and appropriately named, there are a sufficient number of reports of its existence that its adjacent B-tubulin PF be numbered B 10 . Like the B-tubule, the centriolar C-tubule contains 10 tubulin PFs and a gap between its 10th PF and its inner junction with the B-tubule [Tilney et al, 1973]. The work of Wilsman and Furnam [1983] shows a 6-nm gap between PFs B 8 -C 10 , and between A 1 -B 10 , and their Fig.…”

“…Historically, Tilney et al [1973] used tannic-acid treatment/thin-section EM to visualize the PFs of sea urchin (echinoderm) flagellar doublet microtubules and basal body triplet microtubules. Their studies conclusively demonstrated that the A-tubule consists of 13 PFs, and that the B-and Ctubules both consist of 10 PFs.…”

“…1, has a number of compelling advantages. Aside from the historic precedent of the base-totip CCW numbering [Tilney et al, 1973;Linck, 1976;Nojima et al, 1995;Song and Mandelkow, 1995;Nicastro et al, 2006;Setter et al, 2006], a CCW convention permits PF B 10 to lie adjacent to the ephemeral B-11th component, rather than the more confusing CW juxtaposition of PF B 1 and the B-11th component. Even though the B-11th component will eventually be identified and appropriately named, there are a sufficient number of reports of its existence that its adjacent B-tubulin PF be numbered B 10 .…”