Histidine-containing phosphocarrier protein (HPr) is a small cytoplasmic protein that is a component of the phosphoenolpyruvate-dependent sugar phosphotransferase system (PTS). The phosphoenolpyruvate-dependent sugar phosphotransferase system (PTS) is a major carbohydrate transport system in bacteria. The PTS catalyses the phosphorylation of sugar substrates during their translocation across the cell membrane. The mechanism involves the transfer of a phosphoryl group from phosphoenolpyruvate (PEP) via enzyme I (EI) to enzyme II (EII) of the PTS system, which in turn transfers it to a phosphocarrier protein (HPr). In some bacteria HPr is a domain in a larger protein that includes an EIII(Fru) (IIA) domain and in some cases also an EI domain. There is a conserved histidine in the N-terminus of HPr, which serves as an acceptor for the phosphoryl group of EI. In the central part of HPr there is a conserved serine which, in most Gram-positive bacteria and certain Gram-negative bacteria, is phosphorylated by an ATP-dependent protein kinase, a process which probably plays a regulatory role in sugar transport.