Supramolecular Biopolymer Composed of a Doubly (His)₆-Tagged Tandem Z-Domain Conjugated by Zn²⁺ Ions

Synthetic two-dimensional (2D) protein assemblies were engineered using tandem Z-domains derived from the bacterial Protein A. Assembly was induced by introducing hexa-histidine tags to both the N- and C-termini of the tandem Z-domain ((His)₆-(Z)₂-(His)₆) and adding equimolar Zn²⁺ at pH 7. Two lines of evidence suggest preservation of the Z-domain's native structure upon metal-mediated assembly: (i) far-UV circular dichroism spectroscopy; and (ii) selective binding to IgG antibodies, with no detectable interaction with IgA or IgM, consistent with the known specificity of the Z-domain. Scanning transmission electron microscopy demonstrated the formation of 2D protein assemblies exclusively in the presence of Zn²⁺ ions. The widespread use of His-tag engineering and the mild conditions required to assemble (His)₆-(Z)₂-(His)₆ monomers into two-dimensional structures suggest that this approach offers a straightforward and accessible platform for the fabrication of synthetic 2D protein assemblies with potential applications in biotechnology and medicine.