The protein family of membrane-anchored extended synaptotagmin-like proteins (E-Syts) was recently discovered in humans. E-Syt1 to 3 each contain at least one transmembrane domain and three or five C2 domains. To investigate the whole C2 area of murine E-Syt2, highly pure recombinant E-Syt2 (rE-Syt2) covering all three C2 domains was isolated. The structure of rE-Syt2 was studied by small-angle X-ray scattering (SAXS) providing a three-dimensional image of a protein with three C2 domains. Calcium binding of rE-Syt2 triggered structural rearrangements and initiated reversible multimerization of the protein in vitro. Quantitative analysis of the calcium binding revealed an apparent binding constant of 100muM. This is the first structural study of a multi-C2 protein, presumably involved in Ca-dependent signalling events.