FIG. 1.

Synthesis and maturation of PAC in E. coli. The structural pac gene from E. coli ATCC 11105 encodes a 95-kDa polypeptide precursor (preproPAC) composed of, from the N terminus to the C terminus, a signal peptide (S), α subunit (α), connecting peptide (C), and β subunit (β). The signal peptide directs the export of preproPAC into the periplasm and is removed after translocation. Another type of PAC precursor, proPAC, 92 kDa, is formed in the periplasm. Periplasmic processing, which involves a series of proteolytic steps, follows to remove the connecting peptide, and the two subunits (α at 24 kDa and β at 62 kDa) become available for assembly of mature PAC.