BREAKDOWN IN THE RELATIONSHIP BETWEEN THERMAL AND THERMODYNAMIC STABILITY IN AN INTERLEUKIN-1-BETA POINT MUTANT MODIFIED IN A SURFACE LOOP

Sequence variants of the beta-barrel protein interleukin-1beta have, b een analyzed for their stabilities toward irreversible thermal inactiv ation by monitoring the generation of light scattering aggregates on h eating. The derived temperatures for the onset of aggregation (T(agg) values) correlate well with the free energies of unfolding of these pr oteins with the exception of one variant, Lys97 --> Val (K97V), which undergoes aggregation at a temperature 7-degrees-C lower than expected based on its thermodynamic stability. This lower than expected therma l stability may be due to generation of an aggregation-prone unfolding intermediate at a temperature lower than the T(m) of the global trans ition. This hypothesis is supported by the location of residue 97 in t he long 86-99 loop which has structural features suggesting it may com prise a small, independent folding unit or microdomain. The excellent correlation of thermal and thermodynamic stabilities of seven of the e ight variants tested is consistent with accepted models for thermal in activation of proteins. At the same time the poor fit of the K97V vari ant underscores the risk in using thermal stability data in quantitati ve analysis of mutational studies of the folding stability of proteins .