Regulation of the phosphorylation state of the AMPA receptor GluR1 subunitin the postsynaptic density

1. Changes in the phosphorylation state of AMPA-type glutamate receptors ar e thought to underlie activity-dependent synaptic modification. It has been established that the GluR1 subunit is phosphorylated on two distinct sires , Ser-831 and Ser-845, by CaMKII and by PKA, respectively, and that phospho rylation by either kinase correlates with an increase-in the AMPA receptor- mediated current. GluR1 is concentrated in postsynaptic densities and it is expected that this particular receptor pool is involved in synaptic modifi cation. The present study describes the regulation of the phosphorylation s tate of GluR1 in isolated postsynaptic densities. 2. Addition of Ca2+/calmodulin to the postsynaptic density fraction promote s phosphorylation of GluR1, and under these conditions, dephosphorylation i s prevented by the inclusion of phosphatase type 1 inhibitors, microcystin- LR and Inhibitor-1. CaMKII and phosphatase type 1 are also found to be enri ched in the PSD fraction compared to the parent fractions. 3. On the other hand, the addition of cAMP, either by itself or with exogen ous PKA, does not change the phosphorylation slate of GluR1. Prior incubati on of PSDs under dephosphorylating conditions results in only a small PKA-m ediated phosphorylation of GluR1. 4. These results support the hypothesis that PSDs contain the molecular mac hinery to promote the phosphorylation as well as the dephosphorylation of G luR1 on Ser-831, while Ser-845, the site phosphorylated by PKA, appears to be mostly occluded. Thus, it is possible that a large pool of PSD-associate d GluR1 is regulated through modification of the phosphorylation state of t he Ser-831 site only.