Histidine-Rich C-Terminal Tail of Mycobacterial GroEL1 and Its Copper Complex─The Impact of Point Mutations

Beyond copper: examining the significance of His-mutations in mycobacterial GroEL1 HRCT for Ni(II) complex stability and formation

Recently, we have studied the coordination chemistry of the Cu(II)-histidine-rich C-terminal tail (HRCT) complex of the mycobacterial GroEL1 protein. The structure of this domain differs significantly compared to the well-known methionine-glycine-rich GroEL chaperonin - it was predicted that mycobacterial GroEL1 could play a significant role in the metal homeostasis of Mycobacteria, especially copper. However, we found that this particular domain's pattern also repeats in a number of Ni(II)-binding proteins. Here, we present the studies concerning the properties of GroEL1 HRCT as a ligand for Ni(II) ions. For this purpose, we chose eight model peptides: L1 - Ac-DHDHHHGHAH, L2 - Ac-DKPAKAEDHDHHHGHAH, and 6 mutants of the latter in the pH range of 2-11. We examined the stoichiometry, stability, and spectroscopic features of copper complexes. We noticed that similar to the Cu(II)-complex, the presence of a Lys5 residue significantly increases the stability of the system. The impact of His mutations was also examined and carefully studied using NMR spectroscopy. His9 and His13 are the crucial residues for Ni(II) binding, whereas His12 has minimal relevance in complex formation.

Exploring binding preferences: Cu(II), Ni(II), and Zn(II) complexes of mycobacterial GroEL1 His-rich and Glu/His-rich domains

Mycobacterial histidine-rich GroEL1 protein significantly differs from the well-known methionine-glycine-rich GroEL chaperonin and most preferably participates in Cu(II) homeostasis. Some GroEL1 proteins, however, do not possess six but only three histidine residues and more acidic residues that can function as binding sites for metal ions. To evaluate the importance of this difference, we examined and compared the properties of GroEL1 His-rich or Glu/His-rich C-terminal domains as ligands for Cu(II), Ni(II), and Zn(II) ions. We studied the stoichiometry, stability, and binding sites of Cu(II)/Ni(II)/Zn(II) complexes of two model peptides: XEN = Ac-DKPEEEEDGHGHAH (M. xenopi) and ABS = Ac-DKPAEEADHGHGHHGHAH (M. abscessus) in the pH range 2-11. In the case of Cu(II), Ni(II), and Zn(II) complexes of XEN and ABS, ABS always formed more stable complexes. For XEN, there seemed to be no preference for Ni(II) or Zn(II) ions. In contrast, for ABS, Zn(II) formed a complex that was slightly more stable than the one formed by Ni(II). This may be due to the 6 His residues, which preferentially interact with Zn(II) rather than Ni(II). The study identified that an equilibrium of complexes-known as polymorphism-may occur in ABS complexes. Therefore, distinct sets of histidine residues may be involved in metal binding.

Zn(II) Induces Fibril Formation and Antifungal Activity in Shepherin I, An Antimicrobial Peptide from Capsella bursa-pastoris

Shepherin I is a glycine- and histidine-rich antimicrobial peptide from the root of a shepherd's purse, whose antimicrobial activity was suggested to be enhanced by the presence of Zn(II) ions. We describe Zn(II) and Cu(II) complexes of this peptide, aiming to understand the correlation between their metal binding mode, structure, morphology, and biological activity. We observe a logical sequence of phenomena, each of which is the result of the previous one: (i) Zn(II) coordinates to shepherin I, (ii) causes a structural change, which, in turn, (iii) results in fibril formation. Eventually, this chain of structural changes has a (iv) biological consequence: The shepherin I-Zn(II) fibrils are highly antifungal. What is of particular interest, both fibril formation and strong anticandidal activity are only observed for the shepherin I-Zn(II) complex, linking its structural rearrangement that occurs after metal binding with its morphology and biological activity.