Conformational change in the outer doublet microtubules from sea urchin sperm flagella.

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Abstract
Dark-field microscopy with a high-powered light source revealed that the outer doublet microtubules (DMT) from sea urchin (Pseudocentrotus depressus and Hemicentrotus pulcherrimus) sperm flagella assume helically coiled configurations (Miki-Noumura and Kamiya, 1976). The DMT change shape when the pH or Ca2+ concentration is changed. The DMT assumed a left-handed helical shape with a diameter of 3.7 .+-. 0.5 .mu.m and a pitch of 2.8 .+-. 0.7 .mu.m at pH 7.4 with 0.1 mM CaCl2, 1 mM MgSO4 and 10 mM Tris-HCl. When the pH was raised to 8.3, the helical diameter and pitch decreased to 2.1 .+-. 0.1 .mu.m and 1.3 .+-. 0.3 .mu.m, respectively. This transformation was a rapid and reversible process and was completed within 1 min. Between pH 7.2-8.3, the DMT assumed intermediate shapes. When the Ca2+ concentration was depleted with EGTA [ethylene glycol-bis(.beta.-aminoethyl ether) N,N,N''N'' tetraacetic acid], the helical structure became significantly larger in pitch and diameter. For instance, the diameter was 3.8 .+-. 0.4 .mu.m at pH 8.3 with 1 mM EGTA and 2 mM MgSO4. Using a Ca buffer system, this Ca-induced transformation occurred at a Ca concentration of about 10-7 M. The conformational changes in the DMT may play some role in the bending wave form of flagellar movement.

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