Protein Motifs New Functions for C2 Domains

The originally described C2 domains function as calcium-regulated phospholipid-binding domains. Several examples of proteins that have C2 domains (as identified by sequence similarity) have been found, yet these C2 domains have mutations in key residues that are required for calcium binding. Thus, the question remains regarding what the function of these not-quite-C2 domains are. Fukuda et al. show that the C2A domain of the double C2 protein (Doc) serves to localize this isoform of the Doc proteins to the nucleus. Deletion analysis showed that the C2A domain was essential for nuclear targeting. Sequence analysis showed that the Doc C2A domain contains a basic cluster in the region in what corresponds to the calcium-binding loop 3 of classical C2 domains, as well as lacks conserved calcium-binding residues. Thus, Doc does not bind calcium. Transfer of this basic cluster in to the Doc2β isoform led to accumulation of this chimera in the nucleus and resulted in the loss of calcium-stimulated phospholipid binding. Thus, the C2A domain of Doc2 is not a calcium sensor but serves as a nuclear localization signal. These results emphasize that caution is appropriate when ascribing function based on sequence homology; domains that can be defined by sequence homology may have quite varied functionality.

M. Fukuda, C. Saegusa, E. Kanno, K. Mikoshiba, The C2A domain of double C2 protein contains a functional nuclear localization signal. J. Biol. Chem. 276, 24441-24444 (2001). [Abstract] [Full Text]