Annotation of a New Low Voltage Activated Calcium Channel of Trichoplax adhaerens (Phylum Placozoa)

Abstract

Studying the voltage-gated calcium channels sheds light on the formation of systems responsible for the coupling of sensors and actuators in a living cell. A homologue of 2090 aa lUNKth in the Trichoplax sp. H2 scaffold and an incomplete protein of 1510 aa lUNKth in the Trichoplax adhaerens scaffold were identified on the basis of the data on the voltage-gated calcium channel TCav3 (2063 aa) from the Trichoplax adhaerens cells. The incomplete hypothetical protein is annotated as Cav3-channel. An EEDD selective filter was found for all 3 proteins and the calcium channel core structure consisting of 24 trans-membrane α-helices was reconstructed. However, the studied proteins demonstrated significant variations in their cytoplasmic domains that indicates the different specialization of Cav3-channels in the signal transduction. So part of the AID motive (alpha-interacting domain) and the adjacent potential sensor from the annotated channel have homologies in 25 species of bony fish, and the corresponding region from both other channels in 41 species of bony fish and in 4 species of snakes was found. Significantly, a highly conserved IIS1-S2 loop with the IEHHNQP sequence was identified lower from the AID motif of bony fish, like in Trichoplax, while the homologous IEHHEQP sequence was revealed in snakes, which differs in the negative residue of glutamic acid that is also present in the corresponding proteins of the rat and human. A modular mechanism for the evolution of Cav3-channels by insertions and merging of protein domains that perform various regulatory functions is suggested based on the analysis of primary transcripts and mature proteins.