A conformational switch in the Piccolo C2A domain regulated by alternative splicing

C-2 domains are widespread Ca2+-binding modules. The active zone protein Piccolo ( also known as Aczonin) contains an unusual C(2)A domain that exhibits a low affinity for Ca2+, a Ca2+-induced conformational change and Ca2+-dependent dimerization. We show here that removal of a nine-residue sequence by alternative splicing increases the Ca2+ affinity, abolishes the conformational change and abrogates dimerization of the Piccolo C(2)A domain. The NMR structure of the Ca2+-free long variant provides a structural basis for these different properties of the two splice forms, showing that the nine-residue sequence forms a beta-strand otherwise occupied by a nonspliced sequence. Consequently, Ca2+-binding to the long Piccolo C(2)A domain requires a marked rearrangement of secondary structure that cannot occur for the short variant. These results reveal a novel mechanism of action of C-2 domains and uncover a structural principle that may underlie the alteration of protein function by short alternatively spliced sequences.