Serine phosphorylation of the secreted extracellular domain of APP

The phosphorylation status of full-length APP (FL-APP) and secreted APP (s-APP) was investigated in stably transfected cells. ³²P incorporation was detected in the mature full-length APP both in the absence and presence of phorbol ester. Surprisingly, ³²P-phosphate was incorporated in the secreted ectodomain, and this was stable to treatment of the [³²P]-phospho-s-APP with a large excess of PNGase F, suggesting that N-linked oligosaccharide sites do not account for phosphate incorporation. Phosphoamino acid analysis of the [³²P]-phospho-s-APP resulted in the recovery of [³²P]- phosphoserine as the preponderant species. Brefeldin A completely inhibited the release of [³²P]-phospho s-APP, but did not inhibit the incorporation of ³²P into the FL-APP, suggesting that phosphorylation occurs early in the central vacuolar pathway. It is possible that ectodomain phosphorylation by a novel luminal or extracellular protein kinase may play a role in regulating the metabolic fate of APP.