CK2α controls synaptic bouton organization. (A) At wild-type muscle 4 NMJs, synaptic vesicles (DvGlut, red) are clustered into distinct domains within individual synaptic boutons. (B) In CK2α mutants, synaptic vesicle domains were not organized into distinct clusters. (C and D) Presynaptic expression of wild-type (C) but not of kinase-dead CK2α (D) was sufficient to restore synaptic vesicle cluster organization. (E) Quantification of synaptic bouton number on muscle 4 (*, P ≤ 0.05; ***, P ≤ 0.001). (F) Quantification of the number of synaptic vesicle clusters per NMJ. Expression of wild-type but not of kinase-dead CK2α rescued the number of synaptic vesicle clusters. Overexpression of high levels of CK2α in wild-type animals caused a significant 1.7-fold increase in the number of DvGlut clusters compared with controls (***, P ≤ 0.001; n = 48 NMJs). n.s., not significant. (G) Overexpression of CK2α significantly increases the number of satellite boutons and synaptic vesicle clusters. Small clusters of synaptic vesicles often associated with single presynaptic active zones (Brp, green). (H and I) In contrast to controls (H), synapses are no longer organized within uniformly sized synaptic boutons but separated within small satellite boutons (I). Bars, 5 µm. Error bars represent SEM.