Shape of the ciliary doublet microtubule in solution

The individual doublet microtubule (DMT) from Tetrahymena cilia had the appearance of a circular arc in solution and its contour length was around 5 to 6 μm as observed under a dark-field microscope. One end of the circular arc was exactly in focus on the slide under the microscope, whereas the other end was slightly out of focus, suggesting that the circular arc may be regarded as a part of a coiled helix. On measuring the contour length and the end-to-end distance of the arc in solution, we found that the Ca-induced transformation of the ciliary DMT took place at a Ca concentration of about 10^?6m in the medium; that is, the radius of the arc was 3.4 ± 0.9 μm at 10^?3m-Ca and 1.5 ± 0.1 μm at less than 10^?6m-Ca. Change in the pH of the medium brought about no significant difference in the radius of the arc of the DMT. The individual DMT of a negatively stained sample appeared to have a configuration similar to that of the circular arc in solution, bending at right angles to the partition wall between the A and B-tubules. Based on the criterion that the A-tubule is wider than the B-tubule, we found that the A-tubule occurred on the outer side of the circular arc in 90 to 100% of the samples.