REAL-TIME OBSERVATION OF CA2-INDUCED BASAL BODY REORIENTATION IN CHLAMYDOMONAS()

The two basal bodies of Chlamydomonas are connected by a bridge, the d istal fiber, that contains a Ca2+-binding protein, centrin. Although v arious fibrous structures in many organisms containing centrin or simi lar 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+-depende nt contraction, we isolated the flagella-basal body complex from the p aralyzed-flagella mutant pf18 and measured the angle between the two a xonemes at different Ca2+ concentrations. Use of a double mutant with the mutant fa1, deficient in the mechanism for Ca2+-dependent flagella r amputation, enabled the measurement at Ca2+ concentrations greater t han or equal to 10(-4) M. The angle, 80-120 degrees at 10(-9) M Ca2+, was found to decrease by about 20 degrees when the Ca2+ concentration was raised above 10(-6) M. The angle increased again when the Ca2+ con centration was lowered below 10(-7) M. The flagellar apparatuses isola ted from the double mutant between pfl8 and the mutant vfl2 deficient in the structural gene of centrin had an angle of 90-130 degrees at 10 -9 M Ca2+, but the angle did not change when the Ca2+ concentration wa s increased. Thus centrin must be involved in the basal body reorienta tion. In detergent-extracted cell models of the pfl8fa1 mutant, the an gle between the two axonemes was found to decrease transiently by abou t 15 degrees upon iontophoretic application of Ca2+. Hence, the Ca2+-i nduced basal body reorientation can take place even when the basal bod y is contained in the cell body covered by the cell wall. It may funct ion as part of the mechanism for phobic responses wherein Chlamydomona s cells swim backward transiently upon reception of strong light or me chanical stimuli. (C) 1998 Wiley-Liss, Inc.