Signal perception and transduction of gravitaxis in the flagellate Euglena gracilis

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Summary

The motile flagellate Euglena gracilis orients itself in its natural environment by means of gravitaxis and phototaxis. Until recently the mechanism of gravitaxis was not understood. Current results indicate that the density difference between cell body and surrounding medium causes sedimentation of the cell body, resulting in a stretching of the lower membrane. This activates stretch-sensitive ion channels, which leads to a modulation of the membrane potential. The modulation triggers a flagellar reorientation. In order to test this model we performed a series of experiments regarding the role of different ions in signal transduction. Increasing the external calcium concentration (but not decreasing), addition of a calcium ionophore and changes in the calcium fluxes had a drastic effect on gravitaxis. Changes in the potassium concentration resulted in an even higher inhibition. In addition, direct changes in the membrane potential affected gravitaxis. The obtained results indicate that both potassium and calcium play an important role in gravitaxis.

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