Casein kinase 2 phosphorylates G lu A 1 and regulates its surface expression

Controlling the density of α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic acid receptors ( AMPAR s) at synapses is essential for regulating the strength of excitatory neurotransmission. In particular, the phosphorylation of AMPAR s is important for defining both synaptic expression and intracellular routing of receptors. Phosphorylation is a post‐translational modification known to regulate many cellular events and the C ‐termini of glutamate receptors are important targets. Recently, the first intracellular loop1 region of the G lu A 1 subunit of AMPAR s was reported to regulate synaptic targeting through phosphorylation of S 567 by C a 2+ /calmodulin‐dependent protein kinase II ( C a MKII ). Intriguingly, the loop1 region of all four AMPAR subunits contains many putative phosphorylation sites ( S / T / Y ), leaving the possibility that other kinases may regulate AMPAR surface expression via phosphorylation of the loop regions. To explore this hypothesis, we used in vitro phosphorylation assays with a small panel of purified kinases and found that casein kinase 2 ( CK 2) phosphorylates the G lu A 1 and G lu A 2 loop1 regions, but not G lu A 3 or G lu A 4. Interestingly, when we reduced the endogenous expression of CK 2 using a specific short hairpin RNA against the regulatory subunit CK 2β, we detected a reduction of G lu A 1 surface expression, whereas G lu A 2 was unchanged. Furthermore, we identified S 579 of G lu A 1 as a substrate of CK 2, and the expression of G lu A 1 phosphodeficient mutants in hippocampal neurons displayed reduced surface expression. Therefore, our study identifies CK 2 as a regulator of G lu A 1 surface expression by phosphorylating the intracellular loop1 region.