Evolutionary insights into T-type Ca channel structure, function, and ion selectivity from the homologue.

Four-domain voltage-gated Ca (Ca) channels play fundamental roles in the nervous system, but little is known about when or how their unique properties and cellular roles evolved. Of the three types of metazoan Ca channels, Ca1 (L-type), Ca2 (P/Q-, N- and R-type) and Ca3 (T-type), Ca3 channels are optimized for regulating cellular excitability because of their fast kinetics and low activation voltages. These same properties permit Ca3 channels to drive low-threshold exocytosis in select neurons and neurosecretory cells. Here, we characterize the single T-type calcium channel from (TCa3), an early diverging animal that lacks muscle, neurons, and synapses. Co-immunolocalization using antibodies against TCa3 and neurosecretory cell marker complexin labeled gland cells, which are hypothesized to play roles in paracrine signaling. Cloning and in vitro expression of TCa3 reveals that, despite roughly 600 million years of divergence from other T-type channels, it bears the defining structural and biophysical features of the Ca3 family. We also characterize the channel's cation permeation properties and find that its pore is less selective for Ca over Na compared with the human homologue Ca3.1, yet it exhibits a similar potent block of inward Na current by low external Ca concentrations (i.e., the Ca block effect). A comparison of the permeability features of TCa3 with other cloned channels suggests that Ca block is a locus of evolutionary change in T-type channel cation permeation properties and that mammalian channels distinguish themselves from invertebrate ones by bearing both stronger Ca block and higher Ca selectivity. TCa3 is the most divergent metazoan T-type calcium channel and thus provides an evolutionary perspective on Ca3 channel structure-function properties, ion selectivity, and cellular physiology.