The C2A Domain of Double C2 Protein γ Contains a Functional Nuclear Localization Signal

The C2 domain was originally defined as a homologous domain to the C2 regulatory region of Ca²⁺-dependent protein kinase C and has been identified in more than 50 different signaling molecules. The original C2 domain of protein kinase Cα functions as a Ca²⁺ binding module, and the Ca²⁺ binding to the C2 domain allows translocation of proteins to phospholipid membranes. By contrast, however, some C2 domains do not exhibit Ca²⁺ binding activity because of amino acid substitutions at Ca²⁺-binding sites, and their physiological meanings remain largely unknown. In this study, we discovered an unexpected function of the Ca²⁺-independent C2A domain of double C2 protein γ (Doc2γ) in nuclear localization. Deletion and mutation analyses revealed that the putative Ca²⁺ binding loop 3 of Doc2γ contains six Arg residues (¹⁷⁷RLRRRRR¹⁸³) and that this basic cluster is both necessary and sufficient for nuclear localization of Doc2γ. Because of the presence of the basic cluster, the C2A domain of Doc2γ did not show Ca²⁺-dependent phospholipid binding activity. Our findings indicate that by changing the nature of the putative Ca²⁺ binding loops the C2 domain has more diversified function in cellular signaling than a simple Ca²⁺ binding motif.