Effects of Divalent Metals on Nanoscopic Fiber Formation and Small Molecule Recognition of Helical Proteins

Abstract

AbstractMetal dependent protein‐based assemblies derived from the cartilage oligomeric matrix protein (C) coiled‐coil domain (His6‐C) and two variants with mutation at position 40 (His6‐T40A) and 44 (His6‐L44A) are explored. All proteins have an N‐terminal hexahistidine tag (His6) 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, His6‐L44A undergoes a conformational switch from unstructured to α‐helix in the presence of Zn(II). Both His6‐C and His6‐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 His6‐C and His6‐T40A or aggregated structures as observed for His6‐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.