Auto-regulated Protein Assembly on a Supramolecular Scaffold.

Controlled protein assembly provides a means to regulate function. Supramolecular building blocks, including rigid macrocycles, are versatile triggers of protein assembly. Now it is shown that sulfonato-calix[8]arene (sclx8 ) mediates the formation of cytochrome c tetramers in solution. This tetramer spontaneously disassembles at ≥2 equivalents of sclx8 , providing a remarkable example of auto-regulation. Using X-ray crystallography the sclx8 binding sites on cytochrome c were characterized. Crystal structures at different protein-ligand ratios reveal varying degrees (up to 35 %) of protein surface coverage by the flexible calixarene and suggest a mechanism for oligomer disassembly. The solution structure of the oligomer was characterized by small-angle X-ray scattering. Overall, the data indicate calixarene-controlled protein assembly and disassembly without the requirement for a competitive inhibitor, and point to protein encapsulation by a flexible macrocycle.