Plays a central role in cell polarity establishment, and contributes to the organization of zonula adherens, epithelial morphogenesis, and tissue growth (PubMed:2344615, PubMed:12900452, PubMed:10102271, PubMed:11740560). Participates in the assembly, positioning and maintenance of adherens junctions via its interaction with the SAC complex (PubMed:11740560, PubMed:12900452, PubMed:10102271, PubMed:11076972). Controls the coalescence of the spots of zonula adherens (ZA) into an adhesive ring around the cells (PubMed:11740560). It may act as a signal (PubMed:2344615). Involved in morphogenesis of the photoreceptor rhabdomere, for positioning and growth of rhabdomere and AJ during the crucial period of photoreceptor extension along the proximodistal axis of the retina (PubMed:12900452). In cooperation with aPKC, required for the formation and maintenance of the rhabdomeric and stalk apical cortical membrane domains (PubMed:32579558). Recruits aPKC to the apical stalk membrane where it phosphorylates and displaces rhabdomere specifying proteins such as PIP82, to restrict their localization and thus activity to the rhabdomeric apical domain (PubMed:32579558). Component of the crb-galla-Xpd (CGX) complex which is essential for proper mitotic chromosome segregation in early embryos (PubMed:25065591). The CGX complex is also required for cell proliferation in developing wing disks (PubMed:25065591). In the CGX complex, acts with galla-1 or galla-2 to recruit Xpd and thus form the functional complex. Together with apn, plays a key role in trachea development at larval stages (PubMed:30645584). Component of the SAC complex, a complex composed of crb, Patj and sdt (PubMed:11740560, PubMed:10102271, PubMed:11076972). May interact with the par-6 complex, which is composed of par-6, baz and aPKC, via its interaction with Patj (PubMed:12900452, PubMed:10102271, PubMed:11076972). Interacts with other proteins with Patj and sdt via its short cytoplasmic tail (PubMed:11740560). Component of the CGX complex composed of crb, galla (galla-1 or galla-2) and Xpd (PubMed:25065591). Able to interact independently (via intracellular domain) with galla-1, galla-2 and Xpd (PubMed:25065591). Interacts with apn (PubMed:30645584). Specifically localized to the apical membrane (PubMed:11076972, PubMed:2344615, PubMed:32579558). Expressed in a mesh punctate pattern that overlaps with metaphase spindle microtubules (PubMed:25065591). In tracheal cells, colocalizes with apn in the subapical region, a small region of the apical membrane apical to the adherens junctions (PubMed:30645584). In photoreceptor cells, enriched in the apical stalk membrane with some residual expression in the rhabdomere membrane (PubMed:32579558). Expressed in the larval trachea (at protein level). Phosphorylated in the cytoplasmic domain. Flies show severe disruptions in the organization of ectodermally derived epithelia and leading in some cases to cell death in these tissues (PubMed:2344615). RNAi-mediated knockdown in the whole body or tracheae results in depletion of its binding partner sdt, twisted tracheal tubes, lack of gas filling and reduced apical surfaces of tracheal tube cells with death occurring at larval stage 2 or 3 (PubMed:30645584). RNAi-mediated knockdown in germline cells results in severe chromosomal and spindle microtubule defects such as chromosome bridges, bent chromosomes, monopolar spindles and fusion with one or more neighboring spindles (PubMed:25065591). Belongs to the Crumbs protein family.