Control of reactivation and microtubule sliding by calcium, strontium, and barium in detergent-extracted macrocilia ofBeroë

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Abstract
Macrocilia of the ctenophore Beroë are activated to beat continuously in the normal direction by membrane-mediated Ca2+ influx (Tamm: Journal of Comparative Physiology [A] 163:23–31, 1988a). Using saponin or Brij-58 permeabilized models of macrocilia, we show that ATP-reactivation of beating requires μM levels of free Ca2+, Ba2+, or Sr2+. Isolated macrocilia beat initially in reactivation solution (RS) containing Ca2+, Ba2+, or Sr2+ and then undergo microtubule sliding disintegration without added proteases. Addition of protease inhibitors to RS + 10−5 M Ca2+ prevents sliding disruption. Pretreatment in wash solution (containing 1 mM EGTA) without protease inhibitors, followed by RS + 10−5 M Ca2+ with protease inhibitors results in extensive sliding disintegration. However, treatment in wash solution followed by RS + protease inhibitors does not induce sliding. Therefore, Ca2+ is not required for proteolysis by endogenous proteases, but is necessary for sliding disintegration. Local iontophoretic application of Ca2+, Ba2+, or Sr2+ to permeabilized macrocilia in RS lacking these cations triggers motility and/or sliding disintegration. Extrusion of microtubules occurs from the tip or the base, depending on whether or not the macrocilium remains attached to its large actin bundle. Thin sheets of microtubules telescope out initially, due to synchronized sliding of subsets of doublet microtubules from parallel rows of axonemes. Macrocilia are one of the first examples of ATP-induced microtubule sliding which retains Ca2+ sensitivity. In addition, the finding that Ba2+ and Sr2+ also trigger active sliding provides an additional method for investigating the control of dynein-powered microtubule movements.