STRUCTURAL AND FUNCTIONAL CHARACTERIZATION OF WATER-BORNE SIGNALING PROTEIN PHEROMONES FROM THE BIPOLAR CILIATE, EUPLOTES PETZI

Among numerous Antarctic and Arctic species of ciliates that we have stably adapted to grow in laboratory, Euplotes petzi has attracted particular interest because of its strict psychrophilic behavior (it does not tolerate temperatures above 8-9 °C) and capacity to constitutively secrete signaling protein pheromones. Using cultures of genetically different strains, we have isolated and structurally characterized four E. petzi pheromones as small proteins of 32 amino acids including eight cysteines located in strictly conserved positions and predicted to form four intra-chain disulfide bridges. This high density of disulfide bridges would intuitively imply a quite compact globular molecular structure of these molecules. However, this hypothesis appears to be contradicted by nuclear magnetic resonance (NMR) analysis of purified E. petzi pheromone preparations. The NMR protein solution structures show that the regions devoid of regular secondary organization are spatially predominant over regions exhibiting regular -helical organization. Considering that the pheromone structures of Euplotes species living in temperate waters are largely dominated by helical regions, our findings suggest that the E. petzi pheromone cold-adaptation is functionally correlated with an increased flexibility of the molecular backbone and, hence, with a decreased thermo-stability. This hypothesis is currently verified by analyzing the unfolding and refolding properties of E. petzi pheromones when exposed to increased temperatures and variations of other environmental parameters.