Phosphorylation of Alzheimer amyloid precursor protein by protein kinase C

The beta/A4 amyloid precursor protein is a membrane protein with one transmembrane domain. The accumulation and deposition of beta/A4 amyloid protein in Alzheimer's disease is thought to be brought about by altered processing of beta/A4 amyloid precursor protein. Activation of protein kinase C and/or inhibition of protein phosphatases 1 and 2A results in an increase in the proteolytic processing and secretion of beta/A4 amyloid precursor protein. These effects might result either from phosphorylation of beta/A4 amyloid precursor protein by protein kinase C or from phosphorylation of components of the beta/A4 amyloid precursor protein processing apparatus. We have previously reported phosphorylation by protein kinase C of a synthetic peptide corresponding to part of the cytoplasmic domain of beta/A4 amyloid precursor protein. However, it was not known whether beta/A4 amyloid precursor protein holoprotein was phosphorylated in its native conformation in the cell membrane. Using a PC12 (rat pheochromocytoma) semi-intact cell system, we now report that mature isoforms of beta/A4 amyloid precursor protein are phosphorylated by protein kinase C at Ser655. Five COOH-terminal fragments which are generated by processing of mature beta/A4 amyloid precursor protein were also phosphorylated by protein kinase C at Ser655. The results support the idea that the beta/A4 amyloid precursor protein haloprotein is a physiological substrate for protein kinase C. These observations should facilitate our understanding of the relationship between altered protein phosphorylation and beta/A4 amyloid production.