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, been analyzed for their stabilities toward irreversible thermal inactivation by monitoring the generation of light scattering aggregates on heating. The derived temperatures for the onset of aggregation (T(agg) values) correlate well with the free energies of unfolding of these proteins 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 thermal stability may be due to generation of an aggregation-prone unfolding intermediate at a temperature lower than the T(m) of the global transition. This hypothesis is supported by the location of residue 97 in the long 86-99 loop which has structural features suggesting it may comprise a small, independent folding unit or microdomain. The excellent correlation of thermal and thermodynamic stabilities of seven of the eight variants tested is consistent with accepted models for thermal inactivation of proteins. At the same time the poor fit of the K97V variant underscores the risk in using thermal stability data in quantitative analysis of mutational studies of the folding stability of proteins.