Secretion of beta-lactamase into the periplasm of Escherichia coli: evidence for a distinct release step associated with a conformational change.

The secretion of beta-lactamase (EC 3.5.2.6) into the periplasm of Escherichia coli has been followed by pulse-chase labeling at 15 degrees C. Though the periplasmic fraction contains only the mature form of the enzyme, the spheroplast fraction contains the completed precursor and a hitherto undocumented processed form. When whole spheroplasts are treated with trypsin, the processed form in this fraction is completely digested. This is in contrast to the native mature enzyme localized in the periplasm, which is trypsin resistant. The beta-lactamase is evidently processed after translocation to a trypsin-sensitive form that is transiently bound to the periplasmic face of the inner membrane. The release of this processed form into the periplasm occurs concomitantly with a conformational change that results in the soluble, catalytically active, trypsin-resistant structure.