Phosphorylation of the Vesicle Docking Protein p115 Regulates Its Association with the Golgi Membrane

The vesicle docking protein p115 was found to be phosphorylated in a cell cycle-specific manner; it was found phosphorylated in interphase but not in mitotic cells. During interphase, however, two forms of p115 were detected in the cells; the phosphorylated form was found exclusively in cytosol, whereas the unphosphorylated form was associated with membranes, mostly of the Golgi complex. The latter form was released from the membranes upon phosphorylation. Mutational analysis revealed that the phosphorylation site of p115 was the Ser 942 residue in the C-terminal acidic domain. A mutant with a single substitution of Ser 942 → Ala markedly increased its association with the Golgi membrane. Another mutant with Ser 942 → Asp was able to associate with the membrane, although at a decreased level, indicating that the dissociation of p115 from the membrane is not simply due to the negative charge of phosphorylated Ser 942 . Taken together, these results suggest that the phosphorylation of Ser 942 at the C-terminal acidic domain regulates the interaction of p115 with the Golgi membrane, possibly taking part in the regulatory mechanism of vesicular transport.