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 sec retory granules in neurons and neuroendocrine cells, We have already d emonstrated the precursor-product relationship between sulfated SgII a nd four N-terminal derived peptides in GH3B6 prolactin cells. In this study, we have investigated the subcellular compartment in which the c leavage of SgII is initiated by taking advantage of its tyrosine sulfa tion in the trans-Golgi network (TGN). In order to prevent export of r adiosulfated SgII from the TGN, we used brefeldin A (BFA) as well as i ncubation at 20 degrees C. BFA completely inhibited the cleavage of Sg II when added immediately post-pulse. BFA added a few minutes post-pul se or after a 20 degrees C incubation, however, permitted the cleavage of SgII in the presence of the drug. These SgII-derived peptides gene rated in the presence of BFA could not be released upon stimulation of the cells by either thyroliberin, a physiological secretagogue, or KC l. 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-sens itive membrane dynamics.