Phosphoglycerates and protein phosphorylation: identification of a protein substrate as glucose-1,6-bisphosphate synthetase.

We have previously reported the occurrence of two endogenous protein phosphorylation systems in mammalian brain that are enhanced in the presence of 3-phosphoglycerate (3PG) and ATP. We present here a study of one of these systems, the phosphorylation of the 72-kDa protein (3PG-PP72). This system was separated into the substrate, 3PG-PP72, and a kinase by ammonium sulfate fractionation, hydroxyapatite chromatography, and hydrophobic interaction HPLC. The substrate protein was shown to be directly phosphorylated with [1-32P]1,3-bisphosphoglycerate [( 1-32P]1,3BPG) with an apparent Km of 1.1 nM. Nonradioactive 1,3BPG inhibited 32P incorporation in the presence of [gamma-32P]ATP and 3PG. Phosphopeptide mapping and phosphoamino acid analyses indicated that the site of phosphorylation of 3PG-PP72 observed in the presence of 3PG and ATP is a serine residue identical to that observed with [1-32P]1,3BPG. Moreover, [32P]phosphate incorporated into 3PG-PP72 in the presence of 3PG and ATP was removed by subsequent incubation with glucose-1-phosphate or glucose-6-phosphate. Finally, 3PG-PP72 showed chromatographic behaviors identical to those of glucose-1,6-bisphosphate (G1,6P2) synthetase. Based upon these observations, we conclude that 3PG-PP72 is G1,6P2 synthetase and that it is phosphorylated directly by 1,3BPG, which is formed from 3PG and ATP by 3PG kinase present in a crude 3PG-PP72 preparation.