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

Abstract

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 -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 binding and provide a generalized theme for correct identification of the Ca -binding site within this class of molecules. As a common feature, one of the two Ca -binding sites is non-functional in the βγ-crystallin domains of these proteins. While APP-NTD binds Ca with a micromolar affinity which is comparable to DdCAD-1, APP surprisingly does not bind Ca . Crystal structures of APP and Ca -bound APP-NTD reveal that the interface interactions in APP render its Ca -binding site inoperative. Thus, heterodomain association provides a novel mode of Ca -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 . In mechanistic detail, our work demonstrates an interdomain interface adapted to distinct functional niches in APP and its homolog DdCAD-1.

βγ-Crystallination Endows a Novel Bacterial Glycoside Hydrolase 64 with Ca2+-Dependent Activity Modulation

“…A remarkable instance of the domain-derived modulation was observed in abundant perithecial protein (APP), a nonlens homolog from Neurospora. In APP, a canonical Ca 2ϩ -binding site of ␤␥crystallin domain is rendered cryptic owing to interactions with residues from an Ig-like domain (50). As already mentioned, a similar phenomenon is possibly responsible for effecting the Ca 2ϩ -dependent activity modulation in glucanallin.…”

Chemical Properties Determine Solubility and Stability in βγ‐Crystallins of the Eye Lens

“…[196] Unlike related proteins, human βγ-crystallins lack the ability to bind Ca 2 + . [197] Increased divalent cation concentration in the eye lens, which can be caused by aging, [162] smoking, [198,199] or diabetes, [200,201] is associated with lens opacification and cataract formation. Each crystallin protein has idiosyncratic interactions with each metal ion, even among crystallins in the same family and for ions of the same charge and similar size.…”

“…This process is illustrated by the Neurospora crassa abundant perithecial protein (APP), an early crystallin orthologue that is highly aggregation‐resistant at high protein concentrations and lacks functional calcium ion binding sites [196] . Unlike related proteins, human βγ‐crystallins lack the ability to bind Ca 2+ [197] …”