Abstract
Toward understanding physiological events based on ion channel molecules, we systematically surveyed ion channel genes from the recently revealed genome sequence of a tunicate, Ciona intestinalis, which belongs to the protochordates. We found that Ciona contains most sets of voltage-gated channels and transmitter-gated channels that are involved in neuronal signaling. On the other hand, Ciona lacks most of auxiliary subunits such as Nav-beta subunits and MiniK that are important for fine tuning of excitability. Ciona also lacks many channel genes related to sophisticated epithelial ion transport and pain sensation, probably reflecting that the nervous system of noiception and kidney are missing in the protochordate. We also noted that Ciona genome showed unexpectedly large gene diversity in the channel species that are closely related to homeostatic control of ion concentration and body fluid. These include connexins, amiloride-sensitive cation channels, Cl channels and Trp-like channels. We also discovered a novel family of putative membrane proteins that show voltage-dependent gating but no ionic currents. We present that one of these, called CiVsp, contains both modules of voltage sensor and enzyme connected by a short linker sequence, and it exhibits voltage-sensitive modification of enzyme activity through coupling between the two domains in the single molecule. This is the first report of single membrane protein which can transduce electrical signal into chemical signal without requiring ionic flow through membrane. [Jpn J Physiol 54 Suppl:S20 (2004)]
