The ParAs function is realized by two separate proteins in the partitioning system of Myxococcus plasmid pMF1

The par operon in the sole myxobacterial plasmid pMF1 includes a function-unknown parC gene in front of the classical parA and parB genes. Removal of parC severely reduced plasmid stability, but ex-situ compensations of parC did not restore the par system function. Individual expression of parA formed insoluble proteins, while co-expression of parC before parA produced a soluble ParC-ParA heterodimer. ParA alone had no ATPase activity and no polymerization, while ParC addition aided ParA to restore the activities. Fusing ParC and ParA in different ways all produced soluble proteins and some restored ATPase activity or increased plasmid stability. Protein interaction model analysis and experiments revealed that ParC structurally mimics the N-terminal of Ia-type SopA (ParA), endowing the Myxococcus ParA protein to play functions by shifting of ParC between two sites on ParA surface. The present results highlight that ParC functions as a part of ParA to support its soluble expression and function, and the separation of ParC and ParA into two proteins in structure enables the ParC fragment to shift in a larger range around ParA to function during partitioning. Author summaryOur work on ParC here provides a new example for the evolution of multi-domain protein. ParC and ParA are two proteins, but their expression and function act as a whole, which proposes a new regulatory model for bacterial par system, and also provides research ideas and materials for the study of functional coordination and evolution of ParA domains in the future.