Type IV collagen is a major component of basement membranes. We have characterized 11 mutations in emb-9, the α1(IV) collagen gene of Caenorhabditis elegans, that result in a spectrum of phenotypes. Five are substitutions of glycines in the Gly-X-Y domain and cause semidominant, temperature-sensitive lethality at the twofold stage of embryogenesis. One is a glycine substitution that causes recessive, non–temperature-sensitive larval lethality. Three putative null alleles, two nonsense mutations and a deletion, all cause recessive, non–temperature-sensitive lethality at the threefold stage of embryogenesis. The less severe null phenotype indicates that glycine substitution containing mutant chains dominantly interfere with the function of other molecules. The emb-9 null mutants do not stain with anti–EMB-9 antisera and show intracellular accumulation of the α2(IV) chain, LET-2, indicating that LET-2 assembly and/or secretion requires EMB-9. Glycine substitutions in either EMB-9 or LET-2 cause intracellular accumulation of both chains. The degree of intracellular accumulation differs depending on the allele and temperature and correlates with the severity of the phenotype. Temperature sensitivity appears to result from reduced assembly/secretion of type IV collagen, not defective function in the basement membrane. Because the dominant interference of glycine substitution mutations is maximal when type IV collagen secretion is totally blocked, this interference appears to occur intracellularly, rather than in the basement membrane. We suggest that the nature of dominant interference caused by mutations in type IV collagen is different than that caused by mutations in fibrillar collagens.

Type IV collagen in Caenorhabditis elegans is produced by two essential genes, emb-9 and let-2, which encode α1- and α2-like chains, respectively. The distribution of EMB-9 and LET-2 chains has been characterized using chain-specific antisera. The chains colocalize, suggesting that they may function in a single heterotrimeric collagen molecule. Type IV collagen is detected in all basement membranes except those 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 membranes in C. elegans. Using lacZ/green fluorescent protein (GFP) reporter constructs, both type IV collagen genes were shown to be expressed in the same cells, primarily body wall muscles, and some somatic cells of the gonad. Although the pharynx and intestine are covered with basement membranes that contain type IV collagen, these tissues do not express either type IV collagen gene. Using an epitope-tagged emb-9 construct, we show that type IV collagen made in body wall muscle cells can assemble 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 complete development and be partially fertile. Since type IV collagen secreted from muscle cells only assembles into some of the basement membranes that it has access to, there must be a mechanism regulating its assembly. We propose that interaction with a cell surface–associated molecule(s) is required to facilitate type IV collagen assembly.

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