Phosphorylation of alzheimer amyloid precursor protein by protein kinase C

The β A4 amyloid precursor protein is a membrane protein with one transmembrane domain. ^{14-16,22,27,28,32,33} The accumulation and deposition of β A4 amyloid protein in Alzheimer's disease is thought to be brought about by altered processing of β A4 amyloid precursor protein.^7,9,35,36 Activation of protein kinase C and/or inhibition of protein phosphatases 1 and 2A results in an increase in the proteolytic processing³ and secretion⁴ of β A4 amyloid precursor protein. These effects might result either from phosphorylation of β A4 amyloid precursor protein by protein kinase C or from phosphorylation of components of the β A4 amyloid precursor protein processing apparatus.^3,4,9 We have previously reported phosphorylation by protein kinase C of a synthetic peptide corresponding to part of the cytoplasmic domain of β A4 amyloid precursor protein.¹⁰ However, it was not known whether β 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 β A4 amyloid precursor protein are phosphorylated by protein kinase C at Ser⁶⁵⁵. Five COOH-terminal fragments which are generated by processing of mature β A4 amyloid precursor protein were also phosphorylated by protein kinase C at Ser⁶⁵⁵. The results support the idea that the β 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 β A4 amyloid production.