PAP (
pancreatitis-associated protein) is a 16 kDa
lectin-like
protein, which becomes robustly up-regulated in the pancreatic juice during
acute pancreatitis.
Trypsin cleaves the N-terminus of PAP, which in turn forms insoluble fibrils. PAP and its paralogue, the
pancreatic stone protein, induce bacterial aggregation and, more recently, PAP was shown to bind to the
peptidoglycan of Gram-positive bacteria and exert a direct bactericidal effect. However, the role of N-terminal processing in the antibacterial function of PAP has remained unclear. In the present study, we demonstrate that N-terminal cleavage of PAP by
trypsin at the Arg37-Ile38
peptide bond or by
elastase at the Ser35-Ala36
peptide bond is a prerequisite for binding to the
peptidoglycan of the Gram-positive bacterium Bacillus subtilis. The tryptic site in PAP was also efficiently cleaved by nprE (extracellular neutral
metalloprotease) secreted from B. subtilis.
Trypsin-mediated processing of PAP resulted in the formation of the characteristic insoluble PAP species, whereas
elastase-processed PAP remained soluble. N-terminally processed PAP induced rapid aggregation of B. subtilis without significant bacterial killing. The bacteria-aggregating activities of
trypsin-processed and
elastase-processed PAP were comparable. In contrast with previous reports, the Gram-negative Escherichia coli bacterium was not aggregated. We conclude that N-terminal processing is necessary for the
peptidoglycan binding and bacteria-aggregating activity of PAP and that
trypsin-processed and
elastase-processed forms are functionally equivalent. The observations also extend the
complement of
proteases capable of PAP processing, which now includes trypsins, pancreatic elastases and bacterial
zinc metalloproteases of the
thermolysin type.