| Summary: | The freezing waters of the Antarctic coasts host a variety of protozoan ciliates. Among these, Euplotes species dominate. Representatives of their populations can easily be isolated and expanded into laboratory cultures that are able to reproduce true-to-type virtually indefinitely. They provide excellent experimental material to identify and characterize, structurally and functionally, cold-adapted molecules of eukaryotic origin. In this context, we have focused on a family of small (52 to 63 residues), globular signal proteins (or “pheromones”), that strains of E. nobilii synthesize constitutively and secrete into the extracellular environment. For three members (i. e., En-1, En-2, and En-6) of this protein family, we have determined the 3-D structures in solution by NMR spectroscopy. The comparison of these En structures with those previously determined for the Er pheromone family (produced by E. raikovi, a temperate species that is phylogenetically closely allied to E. nobilii) shows that these protein families have strictly retained a common compact three-helix bundle core, to which the cold-adapted En pheromones have added two unique, extended non-structured regions. One region (spanning 10-12 residues) forms the molecule N-terminal extremity, while the second one (of 8-10 residues) connects helices 1 and 2. Together with increased concentrations of negatively charged side-chains on the molecular surface, these non-structured extensions improve, locally and globally, the flexibility and breathing of the En molecules, and may thus help the effective docking and binding of the En molecules to their target receptor proteins on the cell surface at cold temperature.
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