Structure and Ca2+-Binding Properties of the Tandem C2 Domains of E-Syt2

Contacts between the endoplasmic reticulum and the plasma membrane involve extended synaptotagmins (E-Syts) in mammals or tricalbins in yeast, proteins with multiple C-2 domains. One of the tandem C-2 domains of E-Syt2 is predicted to bind Ca2+, but no Ca2+-dependent function has been attributed to this protein. We have determined the crystal structures of the tandem C-2 domains of E-Syt2 in the absence and presence of Ca2+ and analyzed their Ca2+-binding properties by nuclear magnetic resonance spectroscopy. Our data reveal an unexpected V-shaped structure with a rigid orientation between the two C-2 domains that is not substantially altered by Ca2+. The E-Syt2 C(2)A domain binds up to four Ca2+ ions, whereas the C2B domain does not bind Ca2+. These results suggest that E-Syt2 performs an as yet unidentified Ca2+-dependent function through its C(2)A domain and uncover fundamental differences between the properties of the tandem C-2 domains of E-Syts and synaptotagmins.