Brain beta-spectrin phosphorylation: phosphate analysis and identification of threonine-347 as a heparin-sensitive protein kinase phosphorylation site.

Phosphorylation of brain spectrin was studied by a combination of in vivo and in vitro approaches. Chemical analysis of phosphate groups on electrophoretically purified mouse brain beta-spectrin yielded a stoichiometry of 3.2 +/- 0.18 mol of PO4/mol of beta-spectrin. The spectrin isolated by chromatographic methods from mouse brain, pig brain, and human erythrocytes yielded 4.1, 5.6, and 3.2 mol of PO4/mol of spectrin heterodimer, respectively. The 32P labeling of spectrin in retinal ganglion cell neurons or NB 2a/d1 neuroblastoma cells with [32P]orthophosphate showed phosphorylation of only beta-spectrin in vivo. Two-dimensional phosphopeptide map analyses showed that most of the in vivo sites on beta-spectrin were phosphorylated by either a heparin-sensitive endogenous cytoskeleton-associated protein kinase or protein kinase A. Phosphoamino acid analysis of in vivo and in vitro phosphorylated beta-spectrin showed that [32P]phosphate groups were incorporated into both serine (>90%) and threonine residues. In vitro, phosphate groups were incorporated into threonine residues by the heparin-sensitive endogenous protein kinase. The amino acid sequence VQQQLQAFNTY of an alpha-chymotryptic 32P-labeled peptide phosphorylated by the heparin-sensitive cytoskeleton-associated endogenous protein kinase corresponded to amino acid residues 338-348 on the beta1 repeat of beta-spectrinG (betaSPIIa) gene. These data suggest that phosphorylation of Thr347, which is localized on the presumptive synapsin I binding domain of beta-spectrinG, may play a role in synaptic function by regulating the binding of spectrin to synaptic vesicles.