Membrane traffic and Ca²⁺ signals in ciliates

AbstractA Paramecium cell has as many types of membrane interactions as mammalian cells, as established with monoclonal antibodies by R. Allen and A. Fok. Since then, we have identified key players, such as SNARE proteins, Ca2+‐regulating proteins, including Ca2+‐channels, Ca2+‐pumps, Ca2+‐binding proteins of different affinity, etc., at the molecular level, probed their function and localized them at the light and electron microscopy level. SNARE proteins, in conjunction with a synaptotagmin‐like Ca2+‐sensor protein, mediate membrane fusion. This interaction is additionally regulated by monomeric GTPases whose spectrum in Tetrahymena and Paramecium has been established by A. Turkewitz. As known from mammalian cells, GTPases are activated on membranes in conjunction with lumenal acidification by an H+‐ATPase. For these complex molecules, we found in Paramecium an unsurpassed number of 17 a‐subunit paralogs which connect the polymeric head and basis part, V1 and V0. (This multitude may reflect different local functional requirements.) Together with plasmalemmal Ca2+‐influx channels, locally enriched intracellular InsP3‐type (InsP3R, mainly in osmoregulatory system) and ryanodine rec