Interface interactions between βγ‐crystallin domain and Ig‐like domain render Ca 2+ 2+ ‐binding site inoperative in abundant perithecial protein of Neurospora crassa Neurospora crassa

摘要

Summary We describe a set of proteins in which a βγ‐crystallin domain pairs with an Ig‐like domain, and which are confined to microbes, like bacteria, slime molds and fungi. DdCAD‐1 (Ca 2+ ‐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 Ca 2+ binding and provide a generalized theme for correct identification of the Ca 2+ ‐binding site within this class of molecules. As a common feature, one of the two Ca 2+ ‐binding sites is non‐functional in the βγ‐crystallin domains of these proteins. While APP‐NTD binds Ca 2+ with a micromolar affinity which is comparable to DdCAD‐1, APP surprisingly does not bind Ca 2+ . Crystal structures of APP and Ca 2+ ‐bound APP‐NTD reveal that the interface interactions in APP render its Ca 2+ ‐binding site inoperative. Thus, heterodomain association provides a novel mode of Ca 2+ ‐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 βγ‐crystallin domain to bind Ca 2+ . In mechanistic detail, our work demonstrates an interdomain interface adapted to distinct functional niches in APP and its homolog DdCAD‐1.