STILLMAN, J.H.; SOMERO, G.N.: A Comparative Analysis of Protein Thermal Stability in Porcelain Crabs

Kinetic properties of enzymes typically correlate with environmental temperatures in species adapted to different temperatures, but correlation between adaptation temperature and enzyme thermal stability is less clear. Although enzyme thermal stability is related chiefly to primary structure, thermal stability can also be altered by extrinsic factors present in the intracellular milieu. Here, we present a comparative analysis of lactate dehydrogenase (LDH) thermal stability in 22 congeneric species of porcelain crabs from a broad range of thermal habitats. Diversity of LDH stability is high: temperatures required for 50% loss of activity in 10 min ranged from 65� to 75�C, corresponding to half-lives of < 1 min to > 3hrs at 70�C. Although stability correlates with maximal habitat temperature in some sister taxa, phylogenetic comparative analysis incorporating all 22 species does not indicate that the interspecific diversity of LDH stability represents an adaptive response to current thermal habitats. Additionally, we found that LDH stability was not affected by thermal acclimation in several species. Examination of the mechanistic bases of LDH stabilization indicates that differences in stability are related to properties of the LDH molecule itself (intrinsic stability) and to the effects of extrinsic protein(s). Intrinsic differences were shown by the unfolding of structure during heating, as measured by circular dichroism spectroscopy. Stabilizing effects of extrinsic proteins are implied by results of cellular fractionation experiments. We conclude that overall structural stability and functional properties of proteins can evolve independently, and that in vivo protein-protein interactions can provide another means to selectively regulate protein stability.