Dityrosine formation in calmodulin: conditions for intermolecular cross-linking

Area of Science:

• Biochemistry
• Protein Chemistry
• Spectroscopy

Background:

• Calmodulin (CaM) is a crucial calcium-binding protein involved in numerous cellular processes.
• Photoactivated dityrosine cross-linking is a modification that can alter protein structure and function.
• Superoxide dismutase (SOD) is an enzyme that neutralizes superoxide radicals.

Purpose of the Study:

• To investigate the effect of superoxide dismutase on photoactivated dityrosine formation in bovine brain calmodulin.
• To characterize the structure and function of calmodulin dimers formed under specific UV irradiation conditions.

Main Methods:

• UV irradiation of calmodulin in the presence and absence of superoxide dismutase and Ca2+.
• Protein fractionation and purification techniques (sedimentation equilibrium, electrophoresis).
• Chemical characterization of cross-links, enzyme binding assays, fluorescence anisotropy, and sedimentation velocity experiments.

Main Results:

• Superoxide dismutase significantly alters dityrosine cross-linking patterns, favoring dimer formation over monomer cross-linking, especially in the absence of Ca2+.
• The purified calmodulin dimer, though homogeneous in molecular weight, exhibits distinct electrophoretic properties and contains intermolecular cross-links (Tyr-99 to Tyr-138 and Tyr-99 to Tyr-99).
• The dimer binds smooth muscle myosin light chain kinase similarly to native calmodulin, shows comparable catalytic activity, and possesses altered Ca2+ binding characteristics and an elongated structure.

Conclusions:

• Superoxide dismutase plays a critical role in modulating UV-induced dityrosine cross-linking in calmodulin, leading to the formation of functional dimers.
• The dimeric calmodulin exhibits structural and Ca2+ binding properties distinct from the monomer but retains functional similarity in enzyme interactions.
• These findings provide insights into the structural plasticity of calmodulin and the impact of oxidative stress on its function.