Real-time observation of Ca²⁺-induced basal body reorientation in Chlamydomonas

The two basal bodies of Chlamydomonas are connected by a bridge, the distal fiber, that contains a Ca²⁺-binding protein, centrin. Although various fibrous structures in many organisms containing centrin or similar proteins have been shown to contract at Ca²⁺ concentrations > 10^-7-10^{- 6} M, the contractility of the distal fiber in Chlamydomonas has not been demonstrated. To determine whether it undergoes Ca²⁺-dependent contraction, we isolated the flagella-basal body complex from the paralyzed-flagella mutant pf18 and measured the angle between the two axonemes at different Ca²⁺ concentrations. Use of a double mutant with the mutant faI, deficient in the mechanism for Ca²⁺-dependent flagellar amputation, enabled the measurement at Ca²⁺ concentrations ≤ 10^-4 M. The angle, 80-120°at 10^{- 9} M Ca²⁺, was found to decrease by about 20°when the Ca²⁺ concentration was raised above 10^-6 M. The angle increased again when the Ca²⁺ concentration was lowered below 10^-7 M. The flagellar apparatuses isolated from the double mutant between pfl8 and the mutant vfl2 deficient in the structural gene of centrin had an angle of 90-130°at 10^-9 M Ca²⁺, but the angle did not change when the Ca²⁺ concentration was increased. Thus centrin must be involved in the basal body reorientation. In detergent- extracted cell models of the pfl8fal mutant, the angle between the two axonemes was found to decrease transiently by about 15°upon iontophoretic application of Ca²⁺. Hence, the Ca²⁺-induced basal body reorientation can take place even when the basal body is contained in the cell body covered by the cell wall. It may function as part of the mechanism for phobic responses wherein Chlamydomonas cells swim backward transiently upon reception of strong light or mechanical stimuli.