A COOPERATIVE MODEL FOR RECEPTOR RECOGNITION AND CELL-ADHESION - EVIDENCE FROM THE MOLECULAR PACKING IN THE 1.6-ANGSTROM CRYSTAL-STRUCTURE OF THE PHEROMONE ER-1 FROM THE CILIATED PROTOZOAN EUPLOTES-RAIKOVI

The crystal structure of the pheromone Er-1 from the unicellular eukar yotic organism Euplotes raikovi was determined at 1.6 Angstrom resolut ion and refined to a crystallographic R factor of 19.9%. In the tightl y packed crystal, two extensive intermolecular helix-helix interaction s arrange the Er-1 molecules into layers. Since the putative receptor of the pheromone is a membrane-bound protein, whose extracellular C-te rminal domain is identical in amino acid sequence to the soluble phero mone, the interactions found in the crystal may mimic the pheromone-re ceptor interactions as they occur on a cell surface. Based on this, we propose a model for the interaction between soluble pheromone molecul es and their receptors. In this model, strong pheromone-receptor bindi ng emerges as a consequence of the cooperative utilization of several weak interactions. The model offers an explanation for the results of binding studies and may also explain the adhesion between cells that o ccurs during mating.