Common metal ion coordination in LIM domain proteins

Area of Science:

• Biochemistry
• Structural Biology
• Molecular Biology

Background:

• The LIM motif is a conserved cysteine- and histidine-rich sequence crucial for gene expression and cell differentiation.
• Understanding the structural basis of LIM domain function is essential for deciphering its role in biological processes.

Purpose of the Study:

• To characterize the structural features and metal-binding properties of LIM domains.
• To investigate the common structural architecture of LIM domain-containing proteins.

Main Methods:

• Bacterial expression and purification of cysteine-rich intestinal protein (CRIP) and a LIM2 fragment of cysteine-rich protein (CRP).
• Biophysical studies including electronic spectroscopy of cobalt (Co(II)) complexes and cadmium-113 nuclear magnetic resonance (113Cd NMR) of cadmium (Cd(II)) complexes.
• Heteronuclear multiple quantum coherence NMR to identify metal-ligand interactions.

Main Results:

• Both CRIP and LIM2 bind two zinc (Zn(II)) ions per molecule, exhibiting similar metal coordination properties.
• Spectroscopic and NMR analyses revealed a conserved ligand field pattern with one tetrathiolate (S4) and one mixed sulfur-nitrogen (S3N1) metal-binding site.
• The S3N1 site involves a histidine residue, and conserved residues (7 cysteines, 1 histidine) likely form these sites.
• Metal binding sequentially populates the S4 site first and stabilizes a folded structure with secondary elements, while apo-proteins are largely unfolded.

Conclusions:

• The LIM motif likely functions as a conserved structural module with common metal ion coordination features.
• Metal binding is critical for the structural integrity and stability of LIM domains.
• The sequential and differential metal exchange kinetics suggest distinct roles for the two metal-binding sites.