C2 domains are regulatory sequence motifs that occur widely in nature. Synaptotagmin I, a synaptic vesicle protein involved in the Ca 2+ regulation of exocytosis, contains two C2 domains, the first of which acts as a Ca 2ũ sensor. We now describe the three-dimensional structure of this C2 domain at 1.9, 6, resolution in both the Ca2ũ-bound and Ca2+-free forms. The C2 polypeptide forms an eight-stranded I~ sandwich constructed around a conserved four-stranded motif designated as a C2 key. Ca 2ũ binds in a cup-shaped depression between two polypeptide loops located at the N-and C-termini of the C2-key motif.