NMR structures of a psychrophilic family of water-borne signal polypeptides isolated from the polar protozoan ciliate, Euplotes nobilii.

A variety of strains of Euplotes nobilii collected from Antarctic and Arctic waters have been found capable to constitutively secrete cell type-specific signal polypeptides of 50 to 63 amino acids (usually referred to as pheromones) in concentrations high enough (approx 50-150 micrograms of protein/liter of cell culture) to carry out NMR determinations of the relative molecular structures. The four determined pheromone structures all show in common a tight conservation of a three-helix bundle core, that is stabilized by four disulfide bonds and ensures a long-lasting integrity of these molecules in the natural environment. On this conserved scaffold, molecule- and family-specific traits can be distinguished. The individual traits appear to be primarily committed to confer specificity to the autocrine (mitogenic) and paracrine (sexual) signaling activity of each pheromone, and are mainly due to variations in the length and regularity of the three helices, as well as in the shape and orientation of the carboxy-terminal tail. On the other hand, the family-specific traits appear to be evolved in functional correlation with cold-adaptation. Most relevant are: (i) the extension of polypeptide segments devoid of regular secondary structures, (ii) a unique distribution of polar and hydrophobic amino acids, (iii) the presence of solvent-exposed clusters of negatively charged amino acid side chains, and (iv) a central role of aromatic residues in anchoring particular regions of the molecular architecture. Overall these cold-adaptive modifications make the psychrophilic pheromone family of E. nobilii an elegant example of how a high level of global stability of the three-dimensional structures may be combined with sufficient levels of local structural plasticity for efficient functioning of environmental signaling molecules at physiologically low temperatures.