Phosphoenolpyruvate-dependent phosphotransferase system of Staphylococcus aureus: factor IIIlac, a trimeric phospho-carrier protein that also acts as a phase transfer catalyst.

Factor IIIlac (FIII) consists of three identical subunits. It could be shown that each of the subunits carries a phosphoryl group upon phosphorylation (P-FIII) with phosphoenolpyruvate (PEP), enzyme I, and histidine-containing phospho-carrier protein (HPr). The phosphoryl group is bound to a histidyl residue in P-FIII. Each subunit of FIII contains four histidyl residues. After tryptic cleavage a peptide was isolated that contained one other histidyl residue besides the active center histidine. By further cleavage of the peptide T-2 with V-8 Staphylococcus aureus protease it could be shown that His-19 in the sequence of the peptide T-2 is the active center histidine. Another peptide (1-38), caused by incomplete tryptic cleavage, could be isolated. It inhibited the phospho-transfer reaction from PEP to the sugar molecule at the step of factor III-enzyme II recognition. It competes with factor III for the binding site of enzyme II, the membrane component. It is a very hydrophobic peptide. This hydrophobic region is buried in factor III. But upon phosphorylation of factor III it is turned out. Thus P-FIII binds to Triton X-100 micelles whereas factor III does not. This conformational change caused by phosphorylation could be shown by proton nuclear magnetic resonance methods [Kalbitzer, H.R., Deutscher, J., Hengstenberg, W., & Rösch, P. (1981) Biochemistry 20, 6178-6185], by circular dichroism spectroscopy, and by the Ouchterlony double-diffusion method. Antibodies against FIII do not precipitate P-FIII.