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Biomolecules 2013, 3(4), 967-985; doi:10.3390/biom3040967

Unfolding Thermodynamics of Cysteine-Rich Proteins and Molecular Thermal-Adaptation of Marine Ciliates

1
Physics Department, Trento University, Via Sommarive 14, Povo (Trento) 38123, Italy
2
Trento Institute for Fundamental Physics and Applications (TIFPA), Via Sommarive 14, Povo (Trento) 38123, Italy
3
International School for Advanced Studies (SISSA/ISAS), Via Bonomea 265, Trieste 34136, Italy
4
Physics and Astronomy Department, Padua University, Via Marzolo 8, Padua 35131, Italy
5
CNR, Institute of Biophysics, Via alla Cascata 56/C, Povo (Trento) 38123, Italy
6
European Centre for Theoretical Nuclear Physics and Related Areas, Strada delle Tabarelle 286, Villazzano (Trento) 38100, Italy
*
Author to whom correspondence should be addressed.
Received: 9 September 2013 / Revised: 28 October 2013 / Accepted: 29 October 2013 / Published: 18 November 2013
(This article belongs to the Special Issue Protein Folding and Misfolding)
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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.
Keywords: protein folding; temperature molecular adaptation; energy landscape theory; circular dichroism protein folding; temperature molecular adaptation; energy landscape theory; circular dichroism
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MDPI and ACS Style

Cazzolli, G.; Škrbić, T.; Guella, G.; Faccioli, P. Unfolding Thermodynamics of Cysteine-Rich Proteins and Molecular Thermal-Adaptation of Marine Ciliates. Biomolecules 2013, 3, 967-985.

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