Annotation of a New Low-Threshold Potential-Dependent Calcium Channel of Trichoplax adhaerens (Phylum Placozoa)

The study of potential-dependent calcium channels sheds light on the formation of systems responsible for the coupling of sensors and actuators in a living cell. Based on data on the potential-sensitive calcium channel TCa_v3 (2063 amino acid residues) from Trichoplax adhaerens cells, homologues of 2090 amino acid residues in the scaffold of Trichoplax sp. H2 and an incomplete polypeptide with a length of 1510 amino acid residues in the scaffold of Trichoplax adhaerens are identified. The latter hypothetical protein is annotated as a Ca_v3 channel. An EEDD selective filter is found for all three proteins and the core structure of the calcium channel consisting of 24 transmembrane α-helices is reconstructed. Nevertheless, the studied proteins differ in cytoplasmic domains, which indicates a different specialization of Ca_v3 channels when conducting a signal into the cell. For example, part of the AID motif (alpha-interacting domain) and the adjacent potential sensor from the annotated channel have homology in 25 species of bony fish, and the corresponding region from other channels in 41 species of bony fish and in 4 species of snakes. Significantly, a highly conserved IIS1-S2 loop with the IEHHNQP sequence is found below the AID motif of bony fish, as in trichoplax; while a homologous IEHHEQP sequence is found in snakes, characterized by a negative glutamic acid residue, which is also present in the corresponding rat and human proteins. Based on the analysis of primary transcripts and mature polypeptides, a modular mechanism for the evolution of Ca_v3 channels is proposed by inserting and combining protein domains performing various regulatory functions.