Plays a critical role as a guanine nucleotide exchange factor (GEF) for CDC42 in several intracellular processes associated with the actin and microtubule cytoskeleton. Regulates the structure of apical junctions in epithelial cells (By similarity). Participates in the normal lumenogenesis of epithelial cell cysts by regulating spindle orientation (By similarity). Plays a role in ciliogenesis (By similarity). May play a role in membrane trafficking between the cell surface and the Golgi (PubMed:14506234). Binds DNM1 via its N-terminal SH3 domains (PubMed:14506234). The C-terminal SH3 domain binds a complex containing actin, tubulin, Hsp70 and actin-regulatory proteins, such as ENAH, EVL, WIRE, CR16, WAVE1 and NAP1L1 (PubMed:14506234). Interacts with FASLG (By similarity). Interacts (via SH3 domain 6) with WASL (By similarity). Interacts (via SH3 domain 6) interacts with ENAH (By similarity). Interacts (via C-terminal domain) with TJP1; required for the apical cell-cell junction localization of DNMBP (By similarity). Localizes to the apical junction, colocalizes with TJP1.