We wish to understand how organ-specific structures assemble during embryonic development. Here, we consider what determines the sub-apical position of the terminal web in the intestine of C. elegans. The terminal web refers to the organelle-depleted intermediate-filament-rich layer of cytoplasm that underlies the apical microvilli of polarized epithelial cells. We demonstrate that: (i) the widely-used monoclonal antibody MH33 reacts (only) with the gut-specific intermediate filament protein encoded by the ifb-2 gene; (ii) IFB-2 protein accumulates near the gut lumen beginning at the lima bean stage of embryogenesis and remains associated with the gut lumen into adulthood, and; (iii), as revealed by immunoelectron microscopy, IFB-2 protein is confined to a discrete circumferential sub-apical layer within the intestinal terminal web; this layer joins directly to the apical junction complexes that connect adjacent gut cells. To investigate what determines the disposition of the IFB-2-containing structure as the terminal web assembles during development, RNAi was used to remove the functions of gene products previously shown to be involved in the overall apico-basal polarity of the developing gut cell. Removal of dlg-1, ajm-1 or hmp-1 function has little effect on the overall position or continuity of the terminal web IFB-2-containing layer. In contrast, removal of the function of the let-413 gene leads to a basolateral expansion of the terminal web, to the point where it can now extend around the entire circumference of the gut cell. LET-413 has previously been shown to be basolaterally located and to prevent the basolateral expansion of several individual apical proteins. In the present context, we conclude that LET-413 is also necessary to maintain the entire terminal-web/brush-border assembly at the apical surface of C. elegans gut cells, a dramatic example of the so-called "fence" function ascribed to epithelial cell junctions. On the other hand, LET-413 is not necessary to establish this apical location during early development.

Affiliations:
- Laboratory of Molecular Biology, NIDDK, NIH, Bethesda, MD 20892-0510, USA
- Department of Biology, University Ghent, B-9000 Ghent, Belgium
- Institut fr Genetik, Heinrich-Heine-Universitt Dsseldorf
- 0225 Dsseldorf, Germany
- Department of Biochemistry and Molecular Biology, University of Calgary, Alberta, Canada T2N 4N1