CK2 maintains the synaptic localization of cytoskeletal proteins. (A–C) Analysis of Nrg distribution at muscle 4 NMJs. The presence of DvGlut in distal boutons and an intact presynaptic membrane indicate stable nerve terminals. (A) At wild-type NMJs, Nrg was present in all synaptic boutons colocalizing with the membrane marker Hrp. (B) At CK2α^P1/TikR mutant NMJs, Nrg levels were significantly reduced throughout the presynaptic nerve terminal and especially at distal boutons. (C) Neuronal expression of Nrg in CK2α^P1/TikR mutants significantly increased presynaptic Nrg levels. (D–F) Analysis of Ank2-L distribution at muscle 4 NMJs. (D) At wild-type NMJs, Ank2-L was present uniformly throughout the presynaptic nerve terminal. (E) At CK2α^P1/TikR mutant NMJs, Ank2-L staining intensities were significantly reduced, especially at the most distal boutons. (F) Neuronal expression of Nrg in CK2α^P1/TikR mutants was not sufficient to restore presynaptic Ank2-L levels. Bars, 5 µm. (G–J) Quantification of presynaptic Nrg (G), Ank2-L (H), DvGlut (I), and FasII (J) protein levels at distal, central, and proximal boutons. Presynaptic expression of wild-type CK2α, but not of kinase-dead CK2α, was sufficient to restore Nrg and Ank2-L levels. Expression of Nrg in the CK2α^P1/TikR mutant significantly increased Nrg but not Ank2-L levels. Presynaptic levels of DvGlut or FasII were not significantly altered in CK2α^P1/TikR mutant animals (*, P ≤ 0.05; **, P ≤ 0.01; ***, P ≤ 0.001; segment A4, muscle 4, n = 6–12 NMJs). (K) Western blot analysis of Ank2-L and Nrg180 protein levels in larval brain extracts of control and CK2α mutant animals. Ank2-L levels were slightly reduced in CK2α^P1/TikR mutants. (L) Neuronal expression of Nrg in CK2α^P1/TikR mutants failed to rescue the impairment in synapse stability (***, P ≤ 0.001; n = 9–15 animals, muscles 1/9 and 2/10). n.s., not significant. Error bars represent SEM.