A schematic overview of the apical membrane domain in C. elegans. The apical membrane domain comprises at least three groups of proteins: the PAR/aPKC complex, CRB-1/Crumbs, and the spectrins (115, 121, 138). The spectrins, SMA-1/beta-heavy-spectrin and SPC-1/alpha-spectrin, appear to be present in all epithelial tissues, where they mediate the attachment of actin to the apical membrane (116, 147). The PAR/aPKC proteins and CRB-1/Crumbs are only found in non-cuticular epithelia such as the intestine and parts of the pharynx (15). The PAR/aPKC complex consists of three proteins, PAR-3, PAR-6, and PKC-3, and constitutes an apical polarity complex in epithelia (124, 125). CRB-1/Crumbs appears to play a redundant role with LET-413 in the polarization of epithelia (95). ERM-1 genetically interacts with SMA-1 and appears to play a structural role in gut lumen formation (159, 160).