Small heat shock protein HSPB5 uses disorder to bind zinc with high affinity

Abstract

Zinc is an essential metal that supports diverse cellular functions. Zinc exerts its biological activity through protein binding, serving as catalytic cofactors and structural stabilizers of many enzymes, transcription factors, and ubiquitin E3 ligases, among others. Despite total cellular zinc concentrations reaching hundreds of micromolar, free zinc levels are tightly buffered. Elevated free zinc promotes protein mismetalation and aggregation. While zinc is redox inert, its cysteine (Cys)- based protein ligands are readily oxidized. Oxidative modification of Cys leads to zinc dissociation and a rapid increase in free zinc. With �3000 proteins in the human zinc proteome, uncontrolled zinc release could be highly deleterious. Metallothioneins buffer zinc under basal conditions, but their resynthesis following oxidative inactivation occurs on the timescale of hours, raising the question of how free zinc is managed in the interim. Histidine, the second most prevalent zinc-coordinating residue, is resistant to oxidative modification. We characterized zinc binding by the small heat shock protein HSPB5 (αB-crystallin), a Cys-free, histidine-rich protein chaperone that responds to cellular stress and found (1) HSPB5 binds zinc with high affinity and rapid reversibility; (2) zinc binding requires the disordered HSPB5 N-terminal region; (3) zinc binding increases HSPB5 disorder; and (4) prolonged zinc exposure promotes formation of assemblies of oligomers crossbridged by zinc. We propose that HSPB5 has evolved specialized zinc-dependent properties distinct among human small HSPs, enabling it to function not only as a protein chaperone but also as a conditional zinc reservoir under oxidative stress.