FIGURE 9.
Aligned amino acid sequences of exon 12B found in protostome Cav3 channel homologs. Exons 12B code for novel extracellular turret residues from the middle of domain II, segment five through the descending pore helix to five amino acids upstream of selectivity filter glutamate (purple) residue, and invariant aspartate residue after it, conserved in all calcium channels (orange residue). 12b exons are flanked by conserved charged residues (red), surrounding a set of variable amino acids, outside of very conserved cysteine (yellow) residues. Exon 12b is ∼52 aa with a penta-cysteine motif (C … CXC … CXC) or more rarely (CXXC … C . . CXC), in some nematodes. Most protostome invertebrates possess a shorter alternative exon 12a turret (see Fig. 8) in addition to longer exon 12b, and these are highlighted and outlined.