Divalent Cations and the Divergence of βγ-Crystallin Function

Area of Science:

• Biochemistry
• Structural Biology
• Evolutionary Biology

Background:

• The βγ-crystallin superfamily includes vertebrate eye lens proteins and microbial calcium-binding proteins, sharing a common double-Greek key domain structure.
• Vertebrate βγ-crystallins are structural proteins, while microbial counterparts bind calcium ions.
• The tunicate Ci-βγ crystallin bridges these functions, binding calcium and residing in a metal-ion-rich sensory organ.

Purpose of the Study:

• To investigate the impact of divalent cations on the stability and aggregation of Ci-βγ crystallin and human γS-crystallin (HγS).
• To explore the evolutionary transition of βγ-crystallins from calcium binding to apo-form stability in the vertebrate lens.

Main Methods:

• Assessing the effect of various divalent cations (Ca2+, Mg2+, Sr2+, Co2+, Mn2+, Ni2+, Zn2+, Cu2+) on protein stability.
• Monitoring protein aggregation propensity in the presence of these cations.
• Analyzing cation coordination and binding affinities.

Main Results:

• Ci-βγ crystallin coordinates multiple divalent cations, with Sr2+ binding at a comparable affinity to its preferred ion.
• The stabilizing effect of divalent cations on Ci-βγ structure strongly correlates with ionic radius.
• HγS stability was not improved by any tested cation; some induced aggregation, notably Zn2+, Ni2+, Co2+ (via cysteines), and Cu2+ (via a different site).

Conclusions:

• Divalent cation binding and stabilization of Ci-βγ are influenced by ionic radius, suggesting an evolutionary adaptation.
• HγS exhibits cation-induced aggregation, highlighting differences in structural stability and metal ion interaction compared to Ci-βγ.
• Understanding these interactions is crucial for elucidating βγ-crystallin evolution and potential roles in protein stability and function.