Bacillus licheniformis 749/C plasma membrane penicillinase, a hydrophobic polar protein

Plasma membrane penicillinase from Bacillus licheniformis 749/C is hydrophobic in nature, although it is virtually identical to its riydrophilic exoenzyme counterpart in amino acid composition and sequence. Unlike the exoenzyme, however, the purified membrane enzyme retains [ 33P]phosphate and [ 3H]glycerol. By isoelectricfocusing the membrane enzyme is more acidic than the exoenzyme; it has a lower mobility in SDS gel electrophoresis, consistent with the presence of a very hydrophobic moiety. Unlike the exoenzyme, which binds no taurodeoxycholate, the membrane enzyme binds 10 molecules tightly and approximately 37 molecules in the presence of excess taurodeoxycholate (0.1% solution). The membrane enzyme is identical to the exoenzyme in its reaction with antibodies to exopenicillinase as determined by a radioimmune inhibition assay and immunodiffusion in agar. Heat stability studies indicate a slightly less stable conformation for the membrane enzyme, but this difference largely disappears in the presence of antibody to the exoenzyme. Conversion of membrane enzyme to exoenzyme has been achieved by brief treatment with trypsin, or by incubation of impure preparations at pH 9.0 in 25% potassium phosphate. Since the two forms of penicillinase are very similar in conformation, the hydrophobicity of the membrane form of the enzyme would seem to derive from combination with a hydrophobic moiety, probably phospholipid.