TYPE-IV COLLAGEN IS DETECTABLE IN MOST, BUT NOT ALL, BASEMENT-MEMBRANES OF CAENORHABDITIS-ELEGANS AND ASSEMBLES ON TISSUES THAT DO NOT EXPRESS IT

Type IV collagen in Caenorhabditis elegans is produced by two essentia l genes, emb-9 and let-2, which encode alpha 1- and alpha 2-like chain s, respectively. The distribution of EMB-9 and LET-2 chains has been c haracterized using chain-specific antisera. The chains colocalize, sug gesting that they may function in a single heterotrimeric collagen mol ecule. Type IV collagen is detected in all basement membranes except t hose on the pseudocoelomic face of body wall muscle and on the regions of the hypodermis between body wall muscle quadrants, indicating that there are major structural differences between some basement membrane s in C. elegans. Using lacZ/green fluorescent protein (GFP) reporter c onstructs, both type IV collagen genes were shown to be expressed in t he same cells, primarily body wall muscles, and some somatic cells of the gonad. Although the pharynx and intestine are covered with basemen t membranes that contain type IV collagen, these tissues do not expres s either type IV collagen gene. Using an epitope-tagged emb-9 construc t, we show that type IV collagen made in body wall muscle cells can as semble into the pharyngeal, intestinal, and gonadal basement membranes . Additionally, we show that expression of functional type IV collagen only in body wall muscle cells is sufficient for C. elegans to comple te development and be partially fertile. Since type IV collagen secret ed from muscle cells only assembles into some of the basement membrane s that it has access to, there must be a mechanism regulating its asse mbly. We propose that interaction with a cell surface-associated molec ule(s) is required to facilitate type IV collagen assembly.