Interface interactions between βγ-crystallin domain and Ig-like domain render Ca

Area of Science:

• Microbiology
• Structural Biology
• Biochemistry

Background:

• Proteins featuring paired βγ-crystallin and Ig-like domains are found in microbes like bacteria, slime molds, and fungi.
• Examples include Ca2+-dependent cell adhesion molecule-1 (DdCAD-1) and abundant perithecial protein (APP).

Purpose of the Study:

• To elucidate the Ca2+ binding mechanism and regulation in microbial proteins containing βγ-crystallin and Ig-like domains.
• To characterize the structural basis for Ca2+ binding or non-binding in APP and its homolog DdCAD-1.

Main Methods:

• X-ray crystallography was used to determine the structures of APP and Ca2+-bound APP-N-terminal domain (APP-NTD).
• Site-directed mutagenesis was employed to investigate the role of specific interface interactions in Ca2+ binding regulation.
• Biochemical assays were performed to assess Ca2+ binding affinities.

Main Results:

• A generalized theme for identifying Ca2+-binding sites within this protein class was established.
• APP-NTD binds Ca2+ with micromolar affinity, similar to DdCAD-1, but full-length APP does not bind Ca2+.
• Crystal structures revealed that interdomain interface interactions in APP inactivate its Ca2+-binding site, while mutations or domain separation restore Ca2+ binding capability.

Conclusions:

• Heterodomain association offers a novel mechanism for regulating Ca2+ binding in proteins like APP.
• The interdomain interface is adapted for distinct functional roles in APP and DdCAD-1, highlighting functional divergence within this protein family.