Abstract
Euplotes nobilii and Euplotes raikovi are phylogenetically closely allied species of marine ciliates, living in polar and temperate waters, respectively. Their evolutional relation and the sharply different temperatures of their natural environments make them ideal organisms to investigate thermal-adaptation. We perform a comparative study of the thermal unfolding of disulfide-rich protein pheromones produced by these ciliates. Recent circular dichroism (CD) measurements have shown that the two psychrophilic (E. nobilii) and mesophilic (E. raikovi) protein families are characterized by very different melting temperatures, despite their close structural homology. The enhanced thermal stability of the E. raikovi pheromones is realized notwithstanding the fact that these proteins form, as a rule, a smaller number of disulfide bonds. We perform Monte Carlo (MC) simulations in a structure-based coarse-grained (CG) model to show that the higher stability of the E. raikovi pheromones is due to the lower locality of the disulfide bonds, which yields a lower entropy increase in the unfolding process. Our study suggests that the higher stability of the mesophilic E. raikovi phermones is not mainly due to the presence of a strongly hydrophobic core, as it was proposed in the literature. In addition, we argue that the molecular adaptation of these ciliates may have occurred from cold to warm, and not from warm to cold. To provide a testable prediction, we identify a point-mutation of an E. nobilii pheromone that should lead to an unfolding temperature typical of that of E. raikovi pheromones.
Highlights
To survive in a permanently cold environment, psychrophilic microorganisms synthesize proteins that are resistant to cold-induced denaturation and misfolding [1]
We performed a theoretical analysis of protein unfolding/refolding thermodynamics, that complements a recent analysis of circular dichroism (CD) spectra [2] of two closely homologous families of water-borne signaling proteins, which regulate the vegetative growth and sexual mating [3] in two ecologically separated Euplotes species, E. raikovi living in temperate waters and E. nobilii living in polar (Antarctic and Arctic) waters [4]
We show that the unfolding temperatures of these cysteine-rich proteins are strongly influenced by the precise topology of the disulfide bonds in the protein three-dimensional native structure. These conclusions are reached by analyzing the results of an extensive Monte Carlo (MC) simulations of the conformational space sampled by the E. nobilii and E. raikovi pheromones, performed at different temperatures, within a purely native-centric Gō-type model implemented on the coarse-grained (CG) level
Summary
To survive in a permanently cold environment, psychrophilic microorganisms synthesize proteins that are resistant to cold-induced denaturation and misfolding [1]. We show that the unfolding temperatures of these cysteine-rich proteins are strongly influenced by the precise topology of the disulfide bonds in the protein three-dimensional native structure These conclusions are reached by analyzing the results of an extensive Monte Carlo (MC) simulations of the conformational space sampled by the E. nobilii and E. raikovi pheromones, performed at different temperatures, within a purely native-centric Gō-type model implemented on the coarse-grained (CG) level. In this approach, attractive non-bonded interactions are assigned only to the pairs of amino-acids that are in contact in the native state. We find that our theoretical predictions agree well with the corresponding experimental results for the main secondary structural motifs, which were estimated by deconvoluting the whole experimental CD spectra using the DICHROWEB web interface
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