Phosphorylation of the β-Amyloid Precursor Protein at the Cell Surface by Ectocasein Kinases 1 and 2*

The β-amyloid precursor protein (βAPP) is one of the rare proteins known to be phosphorylated within its ectodomain. We have shown previously that βAPP can be phosphorylated within secretory vesicles and at the cell surface (Walter, J., Capell, A., Hung, A. Y., Langen, H., Schnölzer, M., Thinakaran, G., Sisodia, S. S., Selkoe, D. J., and Haass, C. (1997) J. Biol. Chem.272, 1896–1903). We have now specifically characterized the phosphorylation of cell surface-located βAPP and identified two ectoprotein kinases that phosphorylate βAPP at the outer face of the plasma membrane. By using selective protein kinase inhibitors and by investigating the usage of ATP and GTP as cosubstrates, we demonstrate that membrane-bound βAPP as well as secreted forms of βAPP can be phosphorylated by casein kinase (CK) 1- and CK2-like ectoprotein kinases. The ectodomain of βAPP was also phosphorylated by purified CK1 and CK2 in vitro, but not by protein kinases A and C. Phosphorylation of βAPP by ectoprotein kinases and by purified CK1 and CK2 occurred within an acidic domain in the N-terminal half of the protein. Heparin strongly inhibited the phosphorylation of cell-surface βAPP by ecto-CK1 and ecto-CK2, indicating a regulatory role of this extracellular matrix component in βAPP phosphorylation.