PROCESSING OF CHROMOGRANIN-B IN BOVINE ADRENAL-MEDULLA - IDENTIFICATION OF SECRETOLYTIN, THE ENDOGENOUS C-TERMINAL FRAGMENT OF RESIDUES 614-626 WITH ANTIBACTERIAL ACTIVITY

Chromogranins constitute a family of acidic soluble proteins widely di stributed in endocrine cells and neurons. Chromogranin A, the major so luble component in bovine adrenal medullary secretory granules in chro maffin cells, has been shown to be actively processed to peptide fragm ents [Metz-Boutigue, M. Il., Garcia-Sablone, P., Hogue-Angeletti, R. a nd Aunis, D. (1993) Eur. J. Biochem. 217, 247-257]. In the present pap er, the structural features of the proteolytic degradation mechanism o f chromogranin B/secretogranin I have been characterized with regard t o the possible function of this protein as a precursor of biologically active peptides. Chromogranin-B-derived fragments present in bovine c hromaffin granules were identified by microsequencing after separation by two-dimensional gel electrophoresis or high-performance liquid chr omatography. A similar approach was performed to characterize chromogr anin-B-derived fragments released into the extracellular space from de polarized bovine cultured chromaffin cells. In chromogranin B, 18 clea vage sites were identified along the protein chain and chromogranin B/ secretogranin I fragments were generated by proteolytic attack at both the N-terminus and C-terminus. A major fragment corresponding to resi dues 614-626 of the C-terminal sequence, was identified in the extrace llular space; this peptide was found to share sequence and structural similarities with the lytic domain of cecropins and, as expected from this similarity, to display potent antibacterial properties. Endogenou s and synthetic peptides were active on Micrococcus luteus, killing ba cteria in the micromolar concentration range. The synthetic peptide sl ows the growth of Bacillus megaterium and was inactive towards Escheri chia coli. In addition, the synthetic peptide was unable to induce hem olytic activity. This antibacterial function might be of biological si gnificance in the neuroendocrine system of living organisms. We propos e to name this peptide secretolytin.