Analysis of Ni²⁺-induced arrest of Paramecium axonemes

This study examines the molecular basis for paralysis of ciliary motility by Ni²⁺. At concentrations above 0.1 mM, Ni²⁺ slowed and subsequently stopped swimming of living, axenically grown Paramecium tetraurelia. However, some cilia still beat in the presence of 0.1 mM Ni²⁺. When permeabilized and reactivated with 4mM ATP at pCa>7, cells resumed ciliary beat and swam forward at approximately 170±28 μm s^-1; swimming speed increased in the presence of 10 μM cyclic AMP. Addition of Ni²⁺ (pNi<5) caused rapid arrest of all ciliary beat in a single position. This was fully reversible when EGTA was added to raise the pNi. Axonemes were then isolated and sliding was observed in the presence of trypsin and ATP. When pNi was lowered to about 5, sliding was reduced dramatically. This too was reversible with EGTA. Dynein was then extracted from the axonemes and used for in vitro translocation assays. At concentrations of Ni²⁺ where microtubule-sliding and axonemal beat were greatly inhibited or absent, microtubule translocation in vitro by 22S dynein was only slightly affected. However, translocation by 14S dynein was stopped completely. When pNi was raised by repeated washing with solutions containing EGTA, microtubule translocation by 14S dynein resumed. We conclude that Ni²⁺ induces a reversible paralysis by a direct effect on 14S dynein while 22S dynein is not a primary target.