During embryonic development, some isoforms are essential for proper neuronal differentiation and organization. Required for cell polarity; maintenance of apicobasal polarity. Plays a critical role at septate junctions in cellular growth control during larval development. The presence of a guanylate kinase domain suggests involvement in cellular adhesion as well as signal transduction to control cellular proliferation. Cytoskeleton- and membrane-associated. Located at the cytoplasmic face of the membrane in the cellular blastoderm and becomes associated with septate junctions which begin to form between epithelial cells at the time of dorsal closure. In adult flies, located at the apical-lateral membrane boundary of epithelial cells. During the cellular blastoderm stage, isoform B, isoform F, isoform H, isoform I and isoform L expression is localized to the cell borders. From stage 11 onwards, expression is found predominantly in the developing nervous system: axon bundles in the ventral cord and the brain. Stage 14 and 15 embryos exhibit expression in the developing body wall muscle. Expression in neuropil regions of the CNS and at NMJs persists through to larval development. Other isoforms show expression in embryonic epithelial cells. In larvae, expression is seen as a belt around salivary glands, imaginal disks and proventriculus. Expressed in adult reproductive tissues. In epithelia, coexpressed with scrib throughout development. Expressed both maternally and zygotically throughout development. Contains the N-terminal domain essential for correct neuronal development. Contains the N-terminal domain essential for correct neuronal development. Contains the N-terminal domain essential for correct neuronal development. Contains the N-terminal domain essential for correct neuronal development. Contains the N-terminal domain essential for correct neuronal development. Belongs to the MAGUK family. Truncated N-terminus.