Synthesis and function of an O-phosphorylated peptide corresponding to the cell adhesion sequence of bone sialoprotein (BSP).

Bone sialoprotein contains a cell-binding RGD sequence followed by a threonine residue. Since the protein is extensively phosphorylated, this threonine may also be modified. To study whether such a phosphorylation may alter cell-binding properties, the hexapeptide Pro-Arg-Gly-Asp-Thr(O-phosphoryl)-Tyr has been synthesized by the Fmoc technique using benzyl protective groups for Tyr and phosphate, tert-butyl ester for Asp and Pmc for Arg. Removal of Fmoc groups was effected by treatment with 20% morpholine in DMF. The phospho-peptide inhibited binding of R1 cells to BSP-coated surfaces 10 times less efficiently compared with the non-phosphorylated peptide, as did, surprisingly, also the fibronectin-derived peptide Gly-Arg-Gly-Asp-Ser-Pro.