DnaK duplicates and regionally evolves for the increase of proteomic complexity in bacteria

Hsp70 is important for organismic cells to maintain proteostasis and the chaperone protein is duplicated in all eukaryotes and many prokaryotes. Although the functioning mechanism of Hsp70 has been clearly illuminated, the chaperone duplication and functional evolution has been less investigated. DnaK is a highly conserved bacterial Hsp70 family. Here we showed that the dnaK gene is present in 98.9% bacteria and 6.4% bacteria possess duplicated dnaKs; the occurrence and duplication is positively corelated to the increase of proteomic complexity. We identified the interactomes of the two DnaK paralogs in Myxococcus xanthus DK1622, which are mostly nonoverlapped, but both preferring the &{beta} domain proteins. Consistent with the proteomes, the MxDnaK substrates are both significantly size-larger and pI-higher than that of the single E. coli DnaK. MxDnaK1 is heat-shock inducible, prefers to bind cytosolic proteins, while MxDnaK2 is decreased by heat shock, and is more associated with membrane proteins. The nucleotide binding domain and the substrate binding beta domain are responsible for the significant changes of DnaK substrates, and the former also determines the dimerization of MxDnaK2, but not MxDnaK1. Our work highlights that DnaK is duplicated and regionally evolved for the increase of proteomic complexity in bacteria.