Ancestral Sequence Reconstruction and Homology Modeling Link Temperature Adaptation and Conservation of Function With Sequence Evolution in Parvalbumin

Abstract

Temperature is a key factor influencing protein structure and function in poikilotherms. Previous studies of enzymes have shown that orthologs from species acclimated to different thermal niches can maintain a relatively similar level of function at those species' respective physiological temperatures. In some well-characterized enzymes, this conservation of function is correlated with differences in primary structure that lie outside active sites. Information gained from thermal adaptation studies of enzymes can be extended to non-catalytic proteins, which are less thoroughly examined, through the study of parvalbumin structure and function. Parvalbumins are intracellular calcium-binding proteins of the EF-hand type that are thought to function in muscle cells as calcium sinks permitting more rapid unloading of troponin-C, leading to more rapid contraction/relaxation cycles. Parvalbumins contain two functional, highly conserved binding sites and one non-functional site, the AB domain, thought to be an important area of modulation for cation binding. Parvalbumins from teleosts of the sub-order Notothenioid and the unrelated Arctic cod, Boreogadus saida, have converged on a common phenotype. That is, they show similar thermal sensitivity patterns of calcium binding. To explore the underlying structural basis of this similarity in phenotype, we have used ancestral sequence reconstruction combined with homology modeling to identify potential changes in primary structure that have allowed parvalbumins from these disparately related groups of fish to function similarly in their polar habitats. For instance, an Asn to Cys change at position 26 (located in the AB domain) in the evolution of B. saida parvalbumin may lead to the loss of a hydrogen bond in the B-helix. This may provide the change in tertiary structure needed for this parvalbumin to function at polar temperatures.