SURFACE LOCATION OF HPR, A PHOSPHOCARRIER OF THE PHOSPHOENOLPYRUVATE-SUGAR PHOSPHOTRANSFERASE SYSTEM IN STREPTOCOCCUS-SUIS

HPr is a low-molecular-mass phosphocarrier protein of the bacterial ph osphoenolpyruvate (PEP):sugar phosphotransferase system (PTS) found in the cytoplasm or associated with the inner surface of the cytoplasmic membrane, Treatment of Streptococcus suis cells with a Sorvall Omnimi xer, a technique used to extract cell surface components, resulted in the extraction of a major protein with a molecular mass of 9 kDa. Seve ral lines of evidence suggested that this protein was HPr: (i) the S. suis protein showed homology over the first 35 N-terminal amino acid r esidues with the HPrs of Streptococcus salivarius and Streptococcus mu tans, including the signature sequence for the Site of PEP-dependent p hosphorylation; (ii) it cross-reacted with the S. salivarius anti-HPr antibody preparation; (iii) it could be phosphorylated by enzyme I at the expense of PEP, and by a membrane-associated kinase at the expense of ATP; and (iv) it possessed phosphocarrier activity when used as a source of HPr in an in vitro PTS assay. The data suggested that a port ion of the cellular HPr is associated with the external cell surface i n S. suis, a result that was confirmed by immunogold electron microsco py. The cellular HPr of S. suis consisted of two forms that could be d istinguished by the presence or the absence of the N-terminal methioni ne. Amino acid sequence analysis indicated that the cell-surface-assoc iated HPr of S. suis lacked the N-terminal methionine residue.