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

Abstract 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. 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. Competing Interest Statement The authors have declared no competing interest.