GRAVITY PERCEPTION AND SIGNAL-TRANSDUCTION IN SINGLE CELLS

Cellular signal processing in multi-, as well as in unicellular organi sms, has to rely on fundamentally similar mechanisms. Free-living sing le cells often use the gravity vector for their spatial orientation (g ravitaxis) and show distinct gravisensitivities. In this investigation the gravisensitive giant ameboid cell Physarum polycephalum (Myxomyce tes, acellular slime molds) is used. Its gravitaxis and the modulation of its intrinsic rhythmic contraction activity by gravity was demonst rated in 180 degrees-turn experiments and in simulated, as well as in actual, near-weightlessness studies (fast-rotating clinostat; Spacelab D1, IML-1). The stimulus perception was addressed in an IML-2 experim ent, which provided information on the gravireceptor itself by the det ermination of the cell's acceleration-sensitivity threshold. Ground-ba sed experiments designed to elucidate the subsequent steps in signal t ransduction leading to a motor response, suggest that an acceleration stimulus induces changes in the level of second messenger, adenosine 3 ',5'-cyclic monophosphate (cAMP), indicating also that the acceleratio n-stimulus signal transduction chain of Physarum uses an ubiquitous se cond messenger pathway.