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J. Biol. Chem., Vol. 276, Issue 27, 24441-24444, July 6, 2001

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ACCELERATED PUBLICATION
The C2A Domain of Double C2 Protein  Contains a Functional Nuclear Localization Signal^*

Mitsunori Fukuda^§, Chika Saegusa, Eiko Kanno, and Katsuhiko Mikoshiba^¶

From the  Laboratory for Developmental Neurobiology, Brain Science Institute, RIKEN (the Institute of Physical and Chemical Research), 2-1 Hirosawa, Wako, Saitama 351-0198 and the ^¶ Division of Molecular Neurobiology, Department of Basic Medical Science, The Institute of Medical Science, The University of Tokyo, 4-6-1 Shirokanedai, Minato-ku, Tokyo 108-8639, Japan

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 C 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  (Doc2) in nuclear localization. Deletion and mutation analyses revealed that the putative Ca²⁺ binding loop 3 of Doc2 contains six Arg residues (¹⁷⁷RLRRRRR¹⁸³) and that this basic cluster is both necessary and sufficient for nuclear localization of Doc2. Because of the presence of the basic cluster, the C2A domain of Doc2 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.

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