Molecular aspects of calcium signalling at the crossroads of unikont and bikont eukaryote evolution : The ciliated protozoan Paramecium in focus

Abstract

The ciliated protozoan, Paramecium tetraurelia has a high basic Ca2+ leakage rate which is counteracted mainly by export through a contractile vacuole complex, based on its V-type H+-ATPase activity. In addition Paramecium cells dispose of P-type Ca2+-ATPases, i.e. a plasmamembrane and a sarcoplasmic/endoplasmic reticulum Ca2+-ATPase (PMCA, SERCA). Antiporter systems are to be expected, as inferred from indirect evidence. Among the best known cytosolic Ca2+-binding proteins, calmodulin activates Ca2+ influx channels in the somatic cell membrane, but inactivates Ca2+ influx channels in cilia, where it, thus, ends ciliary reversal induced by depolarization via channels in the somatic cell membrane. Centrin inactivates Ca2+ signals after stimulation by its high capacity/low affinity binding sites, whereas its high affinity sites regulate some other functions. Cortical Ca2+ stores (alveolar sacs) are activated during stimulated trichocyst exocytosis and thereby mediate store-operated Ca2+ entry (SOCE). Ca2+ release channels (CRCs) localised to alveoli and underlying SOCE are considered as Ryanodine receptor-like proteins (RyR-LPs) which are members of a CRC family with 6 subfamilies. These also encompass genuine inositol 1,4,5-trisphosphate receptors (IP3Rs) and intermediates between the two channel types. All IP3R/RyR-type CRCs possess six carboxyterminal transmembrane domains (TMD), with a pore domain between TMD 5 and 6, endowed with a characteristic selectivity filter. There are reasons to assume a common ancestor molecule for such channels and diversification further on in evolution. The distinct distribution of specific CRCs in the different vesicles undergoing intracellular trafficking suggests constitutive formation of very locally restricted Ca2+ signals during vesicle–vesicle interaction. In summary, essential steps of Ca2+ signalling already occur at this level of evolution, including an unexpected multitude of CRCs. For dis-/similarities with other bikonts see “Conclusions”.