Investigation of metal interactions with YrpE protein of Bacillus subtilis by a polyhistidine peptide model

The use of model peptides that can simulate the behaviour of a protein domain is a very successful analytical method to study the metal coordination sites in biological systems. Here we study zinc and copper binding ability of the sequence HTHEHSHDHSHAH, which serves as model for the metal interactions with YrpE, a putative metal-binding protein of the ZinT family identified in Bacillus subtilis. Compared to other ZinT proteins secreted by Gram-negative bacteria, the metal-coordination properties of YrpE N-terminal histidine-rich domain have not been yet characterized. Different independent analytical methods, aimed at providing information on the stability and structure of the formed species, have been employed, including potentiometric titrations, electrospray ionization mass spectrometry, UV-Vis spectrophotometry, circular dichroism and electron paramagnetic resonance spectroscopy. The obtained speciation models and equilibrium constants allowed to compare the metal-binding ability of the investigated polyhistidine sequence with that of other well-known histidine-rich peptides. Our thermodynamic results revealed that the YrpE domain HTHEHSHDHSHAH forms more stable metal complexes than other His-rich domains of similar ZinT proteins. Moreover, the studied peptide, containing the alternated (-XH-)_n motif, proved to be even more effective than the His6-tag (widely used in immobilized metal ion affinity chromatography) in binding zinc ions.