Effects of divalent metals on nanoscopic fiber formation and small molecule recognition of helical proteins

Metal dependent protein-based assemblies derived from the cartilage oligomeric matrix protein (C) coiled-coil domain (His ₆-C) and two variants with mutation at position 40 (His ₆-T40A) and 44 (His ₆-L44A) are explored. All proteins have an N-terminal hexahistidine tag (His ₆) that interacts with divalent metal ions Zn(II) and Ni(II). Binding to Zn(II) confers enhanced helical structure and stability, while Ni(II) promotes aggregation. Surprisingly, His ₆-L44A undergoes a conformational switch from unstructured to α-helix in the presence of Zn(II). Both His ₆-C and His ₆-T40A further assemble into discrete nanofibers that appear to be stabilized by Zn(II) in which the fiber formation is dictated by the α-helical content. Because Ni(II) promotes aggregation, the proteins visibly cluster, forming large fiber mats in the case of His ₆-C and His ₆-T40A or aggregated structures as observed for His ₆-L44A. Due to the unique pentameric assembly of the proteins, recognition of a small molecule within the pore is assessed using curcumin as the guest molecule. In the presence of Zn(II), there is enhanced binding to curcumin, while the addition of Ni(II) causes a loss in binding. It is shown that metal binding serves as a trigger to control the conformation of the proteins, affecting the nanoscopic fibrous assemblies and small molecule recognition abilities.