Experimental evolution of promiscuous kin recognition from a homotypic specific cell surface receptor

Recognizing the difference between self and nonself is a crucial step in the development of multicellularity. Myxococcus xanthus is a model organism for studying these processes during the transition from single cell to multicellular life. The polymorphic cell surface receptor TraA helps to mediate these transitions by directing cooperative behavior toward kin or clonemates. TraA is a highly specific receptor, capable of recognizing other TraA proteins with identical or nearly identical sequences by homotypic binding, but the molecular basis of recognition is poorly understood. In this study, we designed a targeted TraA library, consisting of thousands of variants, which changed 10 predicted specificity residues. By screening this library, we identified TraA variants with different combinations of substitutions that resulted in altered recognition, often leading to promiscuous TraA-TraA binding. Additionally, we identified key residues that dictate specificity between distant TraA groups and showed that changing these residues altered the recognition specificity. Furthermore, we propose a model to explain how TraA recognition specificity evolved through the generation of intermediate promiscuous variants driven by reward/punishment interactions. Our results highlight the malleable nature of the TraA variable domain involved in specificity, shedding light on the molecular and evolutionary basis of social recognition in M. xanthus.