Independent cadherin–catenin and Bazooka clusters interact to assemble adherens junctions

Proper epithelial structure requires adherens junction (AJ) assembly. In the early Drosophila embryo, AJ assembly depends on Bazooka (Baz; PAR-3), but it is unclear how Baz affects AJ assembly and what precursors are involved. To understand this process at the molecular level, we counted the number of core AJ proteins and Baz proteins at an average spot AJ (SAJ) and determined their dynamics with fluorescence recovery after photobleaching experiments. These data reveal that SAJs are subdivided into Baz clusters and cadherin-catenin clusters with independent protein numbers and dynamics. This independence suggests that precursory cadherin-catenin clusters might form before SAJ assembly. We identify cadherin-catenin clusters forming between apical microvilli. Further analyses show that they form independently of Baz and that Baz functions in repositioning them to apicolateral sites for full SAJ assembly. Our data implicate cell protrusions in initial cadherin-catenin clustering in the Drosophila melanogaster embryo. Then, independent Baz clusters appear to engage the cadherin-catenin clusters to assemble SAJs.