The role of zinc in the stability of the marginally stable IscU scaffold protein

Understanding the factors that determine protein stability is interesting because it directly reflects the evolutionary pressure coming from function and environment. Here, we have combined experimental and computational methods to study the stability of IscU, a bacterial scaffold protein highly conserved in most organisms and an essential component of the iron-sulfur cluster biogenesis pathway. We demonstrate that the effect of zinc and its consequence strongly depend on the sample history. IscU is a marginally stable protein at low ionic strength to the point that undergoes cold denaturation at around -8 degrees C with a corresponding dramatic decrease of enthalpy, which is consistent with the fluxional nature of the protein. Presence of constitutively bound zinc appreciably stabilizes the IscU fold, whereas it may cause protein aggregation when zinc is added back posthumously. We discuss how zinc coordination can be achieved by different side chains spatially available and all competent for tetrahedral coordination. The individual absence of some of these residues can be largely compensated by small local rearrangements of the others. We discuss the potential importance of our findings in vitro for the function in vivo of the protein.