Pepsin-mediated processing of the cytoplasmic histone H2A to strong antimicrobial peptide buforin I

The intestinal epithelium forms a first line of innate host defense by secr etion of proteins with antimicrobial activity against microbial infection. Despite the extensive studies on the antimicrobial host defense in many gas trointestinal tracts, little is known about the antimicrobial defense syste m of the stomach. The potent antimicrobial peptide buforin I, consisting of 39 aa, was isolated recently from the stomach tissue of an Asian toad, Buf o bufo gargarizans, In this study we examined the mechanism of buforin I pr oduction in toad stomach tissue. Buforin I is produced by the action of pep sin isozymes, named pepsin Ca and Cb, cleaving the Tyr(39)-Ala(40) bond of histone H2A. Immunohistochemical analysis revealed that buforin I is presen t extracellularly on the mucosal surface, and unacetylated histone H2A, a p recursor of buforin I, is localized in the cytoplasm of gastric gland cells . Furthermore, Western blot analysis showed that buforin I is also present in the gastric fluids, and immunoelectron microscopy detected localization of the unacetylated histone H2A in the cytoplasmic granules of gastric glan d cells. The distinct subcellular distribution of the unacetylated histone H2A and the detection of the unacetylated buforin I both on the mucosal sur face and in the lumen suggest that buforin I is produced from the cytoplasm ic unacetylated histone H2A secreted into the gastric lumen and subsequentl y processed by pepsins, Our results indicate that buforin I along with peps ins in the vertebrate stomach may contribute to the innate host defense of the stomach against invading microorganisms.