Biosynthesis of secretogranin II in Xenopus intermediate pituitary

Secretogranin II (SgII) is a sulphated secretory protein found in a broad variety of neuroendocrine cells. We have raised an antiserum against SgII to monitor its fate in Xenopus intermediate pituitary. Pulse-chase incubations in combination with immunoprecipitation analysis showed that SgII was synthesised as an 84-kDa precursor protein which was processed to fragments of 69, 54, 34, 21 and 15 kDa. Secretion of these cleavage products was sensitive to the dopamine D2 receptor agonist apomorphine, and thus occurred via the regulated secretory pathway. When cells were treated with the fungal metabolite brefeldin A or with the specific vacuolar H+-ATPase inhibitor bafilomycin A1, the processing of SgII and the release of its cleavage products were strongly inhibited, indicating that its processing commenced in the later compartments of the secretory pathway. Pulse-chase and immunoblot analysis showed that the 21-kDa fragment was the major SgII-derived cleavage and release product, and carried secretoneurin, a highly conserved peptide flanked by potential dibasic processing sites. Hence, SgII is cleaved to a variety of products that are released via the regulated secretory pathway, while secretoneurin does not seem to represent a major end-product of SgII processing in Xenopus intermediate pituitary.