Proteolytic processing of sulfated secretogranin II in the trans-Golgi network of GH3B6 prolactin cells.

Secretogranin II (SgII) is a protein specific to the matrix of the secretory granules in neurons and neuroendocrine cells. We have already demonstrated the precursor-product relationship between sulfated SgII and four N-terminal derived peptides in GH3B6 prolactin cells. In this study, we have investigated the subcellular compartment in which the cleavage of SgII is initiated by taking advantage of its tyrosine sulfation in the trans-Golgi network (TGN). In order to prevent export of radiosulfated SgII from the TGN, we used brefeldin A (BFA) as well as incubation at 20 degrees C. BFA completely inhibited the cleavage of SgII when added immediately post-pulse. BFA added a few minutes post-pulse or after a 20 degrees C incubation, however, permitted the cleavage of SgII in the presence of the drug. These SgII-derived peptides generated in the presence of BFA could not be released upon stimulation of the cells by either thyroliberin, a physiological secretagogue, or KCl. These results demonstrate that SgII can be cleaved in the TGN. They also evidence that the cleavage occurs in a distal compartment of the TGN different from the sulfation site. The transfer of SgII from the sulfation site to this distal compartment of the TGN involves BFA-sensitive membrane dynamics.