Apical membrane morphogenesis along a luminal space may have specific mechanistic and molecular requirements. Analysis of
erm-1, the C. elegans ortholog of the membrane-linker family ERM (ezrin-radixin-moesin), suggests that a cytoskeletal matrix is needed for the morphogenesis of a luminal membrane. Transgenic lines expressing ERM-1 show enrichment of the protein along diverse apical membranes of tubular epithelia which do not secrete cuticle. A germline
erm-1 deletion,
erm-1 RNAi, and
erm-1 overexpression all result in rotational distortion of the tube and cystic deformation of the lumen in these same epithelia. Confocal and TEM analysis of ERM-1-deficient animals suggests that
erm-1 is directly involved in luminal matrix morphogenesis, and not, as previously suggested for ERMs, in apico-basal polarity and junction formation. Mosaic expression of a partially rescuing
erm-1::gfp transgene demonstrates that the generation of an intact luminal surface is dependent on the presence of ERM-1. ERM proteins bind exclusively to cytoplasmic actin at their C-terminus. We found that
act-5, the C. elegans cytoplasmic actin ortholog, phenocopies
erm-1 and interacts with it genetically.
erm-1s actin-dependent function, however, appears to lie in actin organization at the membrane, rather than in the localization of actin to the membrane during apico-basal polarity generation. We furthermore found that
erm-1 interacts genetically with several mutants having defects in apical membraneous and submembraneous proteins, including
sma-1/-H-spectrin, other exc mutants with
erm-1-similar cystic excretory canals, and mutants with defects in basolateral structural proteins. Gain- as well as loss-of-function mutations in these proteins, especially when in combination, can cause tubulogenesis defects and breakdown of the lumen. This supports the hypothesis that the relative dosages of structural molecules are essential for their function in tissue morphogenesis. In the construction of a luminal matrix, any imbalance in molecules building its mechanical support structures may result in collapse of the entire lumen architecture. We are currently using an integrated
erm-1::gfp transgenic line, that delineates luminal surfaces, in a genome-wide RNAi screen to identify additional molecules required for tubulogenesis.