The conserved serine transporter SdaC moonlights to enable self recognition

Cells can use self recognition to achieve cooperative behaviors. Self-recognition genes principally evolve in tandem with partner self-recognition alleles. However, other constraints on protein evolution could exist. Here, we have identified an interaction outside of self-recognition loci that could constrain the sequence variation of a self-recognition protein. We show that during collective swarm expansion in Proteus mirabilis , self-recognition signaling co-opts SdaC, a serine transporter. Serine uptake is crucial for bacterial survival and colonization. Single-residue variants of SdaC reveal that self recognition requires an open conformation of the protein; serine transport is dispensable. A distant ortholog from Escherichia coli is sufficient for self recognition; however, a homologous serine transporter, YhaO, is not. Thus, SdaC couples self recognition and serine transport, likely through a shared molecular interface. Understanding molecular and ecological constraints on self-recognition proteins can provide insights into the evolution of self recognition and emergent collective behaviors. Abstract Figure