Human S100A3 tetramerization propagates Ca(2+)/Zn(2+) binding states.

Area of Science:

• Biochemistry
• Structural Biology
• Calcium-binding proteins

Background:

• S100A3 protein is crucial for human hair cuticle cell structure.
• It forms a homotetramer through citrullination-dependent mechanisms.
• S100A3 possesses unique Ca(2+) and Zn(2+) binding sites.

Purpose of the Study:

• Investigate the structural and functional role of the unique C-terminal Zn(2+)-binding domain in S100A3 homotetramer assembly.
• Elucidate the interplay between Ca(2+) and Zn(2+) binding in S100A3 structure and function.

Main Methods:

• Spectroscopic techniques (e.g., circular dichroism) to assess secondary structure changes.
• Cation binding assays to determine affinities.
• Small-angle X-ray scattering (SAXS) for structural analysis of the tetramer.
• Site-directed mutagenesis (implied by focus on specific residues).

Main Results:

• Both Ca(2+) and Zn(2+) binding reduce S100A3 alpha-helix content and modulate cation affinity.
• Zn(2+) binding accelerates Ca(2+)-dependent tetramerization and causes helix IV unfolding.
• Cation binding affinities are enhanced upon tetramerization.
• SAXS shows the Ca(2+)/Zn(2+)-bound tetramer shares a similar molecular shape with the Ca(2+)-bound form.
• Intra-tetramer communication of cation binding states occurs via helix III repositioning and C-terminal domain rearrangement.

Conclusions:

• The C-terminal Zn(2+)-binding domain plays a critical role in S100A3 homotetramer assembly and regulation.
• Ca(2+) and Zn(2+) cooperatively modulate S100A3 structure and function, influencing its tetramerization.
• Allosteric mechanisms involving helix III and the C-terminal tail mediate signal propagation within the S100A3 homotetramer.