The water-born protein pheromones of the polar protozoan ciliate, Euplotes nobilii: Coding genes and molecular structures

The protozoan ciliate Euplotes nobilii found in Antarctic and Arctic coastal waters relies on secretion of water-soluble cell type-specific signal proteins (pheromones) to regulate its vegetative growth and sexual mating. For three of these psychrophilic pheromones we previously determined the three-dimensional structures by nuclear magnetic resonance (NMR) spectroscopy with protein solutions purified from the natural sources, which led to evidence that their adaptation to cold is primarily achieved by increased flexibility through an extension of regions free of regular secondary structures, and by increased exposure of negative charges on the protein surface. Then we cloned the coding genes of these E. nobilii pheromones from the transcriptionally active cell somatic nucleus (macronucleus) and characterized the full-length sequences. These sequences all contain an open reading frame of 252–285 nucleotides, which is specific for a cytoplasmic pheromone precursor that requires two proteolytic cleavages to remove a signal peptide and a pro segment before release of the mature protein into the extracellular environment. The 5′ and 3′ non-coding regions are two- to three-fold longer than the coding region and appear to be tightly conserved, probably in relation to the inclusion of intron sequences destined to be alternatively removed to play key regulatory roles in the mechanism of the pheromone gene expression.