βγ-Crystallination Endows a Novel Bacterial Glycoside Hydrolase 64 with Ca-Dependent Activity Modulation.

The prokaryotic βγ-crystallins are a large group of uncharacterized domains with Ca-binding motifs. We have observed that a vast number of these domains are found appended to other domains, in particular, the carbohydrate-active enzyme (CAZy) domains. To elucidate the functional significance of these prospective Ca sensors in bacteria and this widespread domain association, we have studied one typical example from , a bacterium known for its ability to produce acetone, butanol, and ethanol through fermentation of several carbohydrates.

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

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.