Reducing Lambda Repressor to the Core

Lambda repressor fragment λ6−85* is one of thefastest folding small protein fragments known to date. We hypothesized that removal of three out of five helices of λ6−85* would further reduce this protein to its smallest folding core. Molecular dynamics simulations singled out two energetically stable reduced structures consisting of only helices 1 and 4 connected by a short glycine/serine linker, as well as a less stable control. We investigated these three polypeptides and their fragments experimentally by using circular dichroism, fluorescence spectroscopy, and temperature jump relaxation spectroscopy to gain insight into their thermodynamic and kinetic properties. Based on the thermal melts, the order of peptide stability was in correspondence with theoretical predictions. The most stable two-helix bundle, λblue1, is a cooperatively folding miniprotein with the same melting temperature and folding rate as the full-length λ6−85* pseudo wild type and a well-defined computed structure.