Synaptic junctional glycoproteins are phosphorylated by cyclic-AMP-dependent protein kinase

Synaptic junctions (SJs) isolated from rat brain are associated with protein kinase activity and a unique complement of high molecular weight gglycoproteins. Incubation of SJs with [gamma-32P]A+ glycoproteins which were retained by concanavalin A agarose (con A+ glycoproteins). Three major (apparent mol. wt. 180 K, 130 K and 110 K) and 2 minor (apparent mol. wt. 230 K and 145 K) glycoproteins were identified in the con A+ fraction. Of these, GP180 incorporated the most 32P and GP145 was not labeled. Peptide mapping experiments showed that each molecular weight class of glycoprotein was associated with a unique set of phosphorylated peptides. Cyclic AMP stimulated the incorporation of 32P into total SJ proteins and con A+ lycoproteins by 38% and 58%, respectively. GP130 showed the greatest increase in labelling in the presence of cyclic AMP (198% of control levels) although incorporation into all 4 glycoproteins was increased. Cyclic AMP selectively stimulated the incorporation of 32P into only 2 of the 6 phosphorylated peptides derived from GP130. These studies demonstrate that endogenous glycoproteins serve as substrates for intrinsic SJ protein kinases and identify this reaction as a potential means of modifying postsynaptic membrane function.