Signal transduction in gravisensing of flagellates

Abstract

Many photosynthetic or heterotrophic flagellates, from various taxa, show positive, negative and, in some cases, transversal gravitaxis. Most flagellates are unicellular; however, they can form aggregates or colonies. Two species have been studied in more detail, Chlamydomonas and Euglena, which can serve as model systems. Earlier theories suggested that gravitaxis is caused by a buoy effect: The cell is tail-heavy, and the propelling flagellum (flagella), inserting at the anterior end, pulls the organism upwards. Recent investigations, however, falsify these hypotheses and indicate the presence of an active, physiological graviperception mechanism, in some cases supported by a passive mechanism. In the photosynthetic Euglena, the cell body is heavier (1.04 g/mL) than the surrounding medium (water) and is assumed to exert pressure onto the lower membrane. The resulting force is believed to trigger mechano-sensitive calcium-specific ion channels. The molecular sensory transduction chain starts with the Ca influx which causes a depolarization of the membrane potential. The calcium is believed to bind to calmodulin which in turn activates an adenylyl cyclase. The produced cAMP is a secondary messenger and finally activates the flagellum reorientation. In Chlamydomonas, a mutant with defective mechano-sensitive channels was found which is agravitactic, indicating that also in this flagellate gravitaxis is mediated by an active physiological receptor.