HPTransfase is a family of essential histidine phosphotransferases. It controls the activity of the master bacterial cell-cycle regulator CtrA through phosphorylation. It behaves as a homodimer by adopting the domain architecture of the intracellular part of class I histidine kinases. Each subunit consists of two distinct domains: an N-terminal helical hairpin domain and a C-terminal [alpha]/[beta] domain. The two N-terminal domains are adjacent within the dimer, forming a four-helix bundle. The C-terminal domain adopts an atypical Bergerat ATP-binding fold.