Calcium signalling in the ciliated protozoan model, Paramecium: Strict signal localisation by epigenetically controlled positioning of different Ca2+-channels.

The Paramecium tetraurelia cell is highly organised, with regularly spaced elements pertinent to Ca 2+ signalling under epigenetic control. Vesicles serving as stationary Ca 2+ stores or undergoing trafficking contain Ca 2+ -release channels ( Pt CRCs) which, according to sequence and domain comparison, are related either to inositol 1,4,5-trisphosphate (InsP 3 ) receptors (IP 3 R) or to ryanodine receptor-like proteins (RyR-LP) or to both, with intermediate characteristics or deviation from conventional domain structure. Six groups of such Pt CRCs have been found. The ryanodine-InsP 3 -receptor homology (RIH) domain is not always recognisable, in contrast to the channel domain with six trans-membrane domains and the pore between transmembrane domain 5 and 6. Two CRC subtypes tested more closely, Pt CRC-II and Pt CRC-IV, with and without an InsP 3 -binding domain, reacted to InsP 3 and to caffeine, respectively, and hence represent IP 3 Rs and RyR-LPs. IP 3 Rs occur in the contractile vacuole complex where they allow for stochastic constitutive Ca 2+ reflux into the cytosol. RyR-LPs are localised to cortical Ca 2+ stores; they are engaged in dense core-secretory vesicle exocytosis by Ca 2+ release, superimposed by Ca 2+ -influx via non-ciliary Ca 2+ -channels. One or two different types of Pt CRCs also occur in other vesicles undergoing trafficking. Since the Pt CRCs described combine different features they are considered derivatives of primitive precursors. The highly regular, epigenetically controlled design of a Paramecium cell allows it to make Ca 2+ available very locally, in a most efficient way, along predetermined trafficking pathways, including regulation of exocytosis, endocytosis, phagocytosis and recycling phenomena. The activity of cilia is also regulated by Ca 2+ , yet independently from any CRCs, by de- and hyperpolarisation of the cell membrane potential. [ABSTRACT FROM AUTHOR]