Protein-folding location can regulate manganese-binding versus copper- or zinc-binding

Area of Science:

• Biochemistry and Molecular Biology
• Cell Biology
• Metalloprotein research

Background:

• Proteins require metals for function, but mechanisms for metal acquisition by most proteins are unknown.
• Metallochaperones are not identified for most metalloproteins, suggesting direct acquisition from cellular pools.
• Metal-protein binding affinities vary, as described by the Irving-Williams series, influencing metal acquisition.

Purpose of the Study:

• To investigate cellular mechanisms controlling metal acquisition by nascent proteins.
• To identify and characterize abundant copper and manganese binding proteins in Synechocystis PCC 6803.
• To elucidate the role of protein folding location in metal content regulation.

Main Methods:

• Identification of Cu(2+)-cupin A (CucA) and Mn(2+)-cupin A (MncA) in Synechocystis PCC 6803.
• Analysis of metal binding properties and stability of MncA and CucA.
• Investigation of protein export pathways (Tat and Sec) and their impact on protein folding and metal acquisition.

Main Results:

• MncA and CucA bind Mn(2+) and Cu(2+) respectively via identical ligands within a cupin fold.
• MncA exhibits specific Mn(2+) binding only after folding in low Cu(2+)/Zn(2+) and high Mn(2+) conditions.
• Protein export pathway dictates folding location (cytoplasm vs. periplasm), overriding metal binding preferences.

Conclusions:

• Protein folding compartment overrides intrinsic metal binding affinity to control metalloprotein composition.
• This mechanism explains the distinct metal buffering requirements of the cytoplasm for copper and zinc.
• The study reveals a novel strategy for cellular metal management in metalloproteins.