Interface interactions between ßγ-crystallin domain and Ig-like domain render Ca2+ -binding site inoperative in abundant perithecial protein of Neurospora crassa.

We describe a set of proteins in which a ßγ-crystallin domain pairs with an Ig-like domain, and which are confined to microbes, like bacteria, slime molds and fungi. DdCAD-1 (Ca2+ -dependent cell adhesion molecule-1) and abundant perithecial protein (APP) represent this class of molecules. Using the crystal structure of APP-NTD (N-terminal domain of APP), we describe its mode of Ca2+ binding and provide a generalized theme for correct identification of the Ca2+ -binding site within this class of molecules. As a common feature, one of the two Ca2+ -binding sites is non-functional in the ßγ-crystallin domains of these proteins. While APP-NTD binds Ca2+ with a micromolar affinity which is comparable to DdCAD-1, APP surprisingly does not bind Ca2+ . Crystal structures of APP and Ca2+ -bound APP-NTD reveal that the interface interactions in APP render its Ca2+ -binding site inoperative. Thus, heterodomain association provides a novel mode of Ca2+ -binding regulation in APP. Breaking the interface interactions (mutating Asp30Ala, Leu132Ala and Ile135Ala) or separation from the Ig-like domain removes the constraints upon the required conformational transition and enables the ßγ-crystallin domain to bind Ca2+ . In mechanistic detail, our work demonstrates an interdomain interface adapted to distinct functional niches in APP and its homolog DdCAD-1.