Most genes encoding cytoplasmic intermediate filament (IF) proteins of the nematode Caenorhabditis elegans are required in late embryogenesis

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Intestinal cells of C. elegans show an unexpectedly high complexity of cytoplasmic intermediate filament (IF) proteins. Of the 11 known IF genes six are coexpressed in the intestine, i.e. genes B2, C1, C2, D1, D2, and E1. Specific antibodies and GFP-promoter constructs show that genes B2, D1, D2, and E1 are exclusively expressed in intestinal cells. Using RNA interference (RNAi) by microinjection at 25 °C rather than at 20 °C we observe for the first time lethal phenotypes for C1 and D2. RNAi at 25 °C also shows that the known A1 phenotype occurs already in the late embryo after microinjection and is also observed by feeding which was not the case at 20 °C. Thus, RNAi at 25 °C may also be useful for the future analysis of other nematode genes. Finally, we show that triple RNAi at 20 °C is necessary for the combinations B2, D1, E1 and B2, D1, D2 to obtain a phenotype. Together with earlier results on genes A1, A2, A3, B1, and C2 RNAi phenotypes are now established for all 11 IF genes except for the A4 gene. RNAi phenotypes except for A2 (early larval lethality) and C2 (adult phenotype) relate to the late embryo. We conclude that in C. elegans cytoplasmic IFs are required for tissue integrity including late embryonic stages. This is in strong contrast to the mouse, where ablation of IF genes apparently does not affect the embryo proper.

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    As expected, all newly identified sequences have a long rod domain (LRD; Table 1 and Fig. S1K) typical for cytoplasmic IF proteins of invertebrates and, except IFC and IFD-2, have also the lamin homology domain (LHD) in their tails (Table 1 and Fig. S1; for review on invertebrate cytoplasmic IFs see Erber et al., 1998). Moreover, the newly identified IFB-1, IFB-2, IFC-1, IFC-2, IFD-2 and IFP-1 protein sequences harbour several previously established unique rod deletions in their corresponding C. elegans counterparts (Fig. S1K; for details see Dodemont et al., 1994; Karabinos et al., 2003a, 2004), which trace them to the common IFB-1, IFB-2, IFC-1, IFC-2, IFD-2 and IFP-1 ancestor, respectively. The same holds, also, for the newly established IFCDP-1 and IFCDP-2 IF sequences, which share with the IFC, IFD and/or IFP-1 proteins the unique deletion in the L12 linker and the coil 1b rod domain (Fig. S1K).

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