Single Cells Signal Non-self from Self via 'XOR' and 'NOT EQUALS' Logic in a Dimer of Dimers

The ciliate Tetrahymena thermophila ’s seven mating types are defined by unique receptor-ligand pairs (Mta/Mtb). While Mta and Mtb are known to participate in a mating signal complex, how they distinguish between oneself and six non-self cell types remains unknown. AlphaFold3 predictions reveal Mta/Mtb as large glycoproteins likely derived from ancient, unisexual, intercellular adhesion molecules. Since homologous binary-type systems perform XOR by switching mtA expression, we show spectrum ( n ) types naturally extend XOR to multi-bit NOT EQUALS operations via differential affinities of Mta/Mtb dimers. We model kinetics begetting the ‘ n + 1th’ type, demonstrating self-inhibition by trans -homophilic Mtb-Mtb. A computational approach reconciles recent and classical evidence for mating exclusivity, including selfing failures (same type mating). Binding kinetics enables fast, robust intercellular computation across an intermembrane mating space. Thus, Mta/Mtb families are a model system allowing us to derive a ‘calculus’ of antigenic variation and inspire synthetic designs of XOR logic underlying self/non-self recognition. Graphical Abstract Highlights Within minutes, swimming cells signal compatible mating types via membrane proteins AlphaFold insights reveal Mta/Mtb family as elongate adhesion receptors/ligands Relative affinities of two dimers explain multi-bit intercellular signaling capacity Dimer-of-dimers compute NOT EQUALS via homo-vs. hetero-philic interactions In Brief Free-living ciliates T. thermophila breed only when two cells are different in mating type, out of seven types total. Their mating-type-specific coexpression of a receptor-ligand glycoprotein pair reveals how competitive homophilic vs. heterophilic binding computes self vs. non-self recognition with single-layer intercellular logic. Proteins perform binary XOR gates (with two mating types) that extend to multi-bit ‘NOT EQUALS’ (with multiple mating types). The work illuminates principles of eXclusivity in molecular recognition and inspires the design of synthetic cellular recognition systems.