SELECTIVE ECTODOMAIN PHOSPHORYLATION AND REGULATED CLEAVAGE OF BETA-AMYLOID PRECURSOR PROTEIN

The beta-amyloid precursor protein (betaAPP) is a highly conserved integral membrane protein expressed in most mammalian tissues and found at highest levels in the nervous system. Cerebral deposition of the amyloid beta-peptide (Abeta), derived by proteolysis of betaAPP, is an early and invariant feature of Alzheimer's disease. Protein phosphorylation by protein kinase C (PKC) has been found to regulate the metabolism of betaAPP into nonamyloidogenic and amyloidogenic derivatives, but both the mechanism of these effects and the nature of betaAPP phosphorylation are unknown. When labeled in vivo with [P-32]orthophosphate, betaAPP was phosphorylated only on serine residues in the N-terminal half of the extracellular domain, resulting in the secretion of phosphorylated soluble betaAPP. PKC-mediated stimulation of betaAPP secretion and concurrent inhibition of Abeta release did not involve enhanced phosphorylation of betaAPP and proceeded in the absence of cytoplasmic or extracellular phosphorylation of the precursor. The region of betaAPP required for this indirect regulation by PKC was largely restricted to a 64 amino acid stretch around the secretory cleavage site. Moreover, in a truncated molecule designed to release soluble betaAPP without the need for proteolytic cleavage, secretion was no longer regulated by PKC. Our data indicate that PKC-mediated pathways play a pivotal role in the control of betaAPP metabolism and amyloid formation. However, in contrast to current postulates, this regulation is independent of betaAPP phosphorylation and instead involves phosphorylation of other substrates that alter betaAPP processing, such as betaAPP-cleaving proteases.