C2 region <p>C2 domains are Ca2+-regulatory domains found widely in nature. To date,more than 40 distinct C2 domains have been identified, for example inproteins such as synaptotagmin, phospholipase A2, GTPase-activatingproteins, rabphilin and phospholipase C isoforms [<cite idref="PUB00000918"/>]. The domains show no sequence similarity to other Ca2+-binding proteins; they thusconstitute a novel family of Ca2+-regulatory motifs, which alsoincludes members that have lost their calcium regulation.</p><p>The structure of C2 domains in synaptotagmin I has been deduced: the C2polypeptide forms an 8-stranded beta-sandwich constructed around a conserved 4-stranded motif, designated a C2 key [<cite idref="PUB00000918"/>]. Calcium binds ina cup-shaped depression formed by the N- and C-terminal loops of theC2-key motif.</p>