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. 2015 May 19;6:153. doi: 10.3389/fphys.2015.00153

Figure 2.

Figure 2

Invertebrate NALCN and T-type channels splice in the pore module (Domain II) to generate calcium- or sodium-selective channels. Only repeat Domains II and IV are shown for clarity. Each repeat domain contains a voltage-sensing module (S1 to S4) and segments S5-P-S6 that contribute a quarter to the pore module. (A) Exons 15a or 15b code for alternative HFS sites generating calcium-selective (EEEE) or sodium-selective (EKEE) pores (Senatore et al., 2013). (B) Expression of exon 12a or 12b upstream of the HFS site in the Cav3 T-Type channels generates alternative ion selectivity in the of the same invertebrate species (giant pond snail Lymnaea stagnalis). The exons code for extracellular loops with three or five cysteines that engender the invertebrate T-type channels with greater sodium selectivity or less sodium selectivity, respectively (Senatore et al., 2014a).