HPr is a low-molecular-mass phosphocarrier protein of the bacterial phosphoenolpyruvate (PEP): sugar phosphotransferase system (PTS) found in the cytoplasm or associated with the inner surface of the cytoplasmic membrane. Treatment of cells with a Sorvall Omnimixer, a technique used to extract cell surface components, resulted in the extraction of a major protein with a molecular mass of 9 kDa. Several lines of evidence suggested that this protein was HPr: (i) the protein showed homology over the first 35 N-terminal amino acid residues with the HPrs of and including the signature sequence for the site of PEP-dependent phosphorylation; (ii) it cross-reacted with the anti-HPr antibody preparation; (iii) it could be phosphorylated by enzyme 1 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 PTS assay. The data suggested that a portion of the cellular HPr is associated with the external cell surface in a result that was confirmed by immunogold electron microscopy. The cellular HPr of consisted of two forms that could be distinguished by the presence or the absence of the N-terminal methionine. Amino acid sequence analysis indicated that the cell-surface-associated HPr of lacked the N-terminal methionine residue.


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