Genetic studies indicate that the let-23 gene affects several developmental decisions in the nematodeCaenorhabditis elegans. let-23 is required for the proper development of the hermaphrodite vulva, the male tail, and the posterior ectoderm. In addition, let-23 mutations can cause lethality and hermaphrodite sterility. These five let-23 functions can be independently mutated, suggesting that the let-23 protein encodes tissuespecific functions . Furthermore, let-23 controls two opposing pathways: one that stimulates and another that inhibits vulvaldevelopment. These two pathways ensure that the proper level of vulval development occurs. Twenty let-23 alleles exist: 14 eliminate function (null), three reduce function in all tissues (hypomorphic), and three reduce function in certain tissues (tissue-specific). In addition, two of these alleles are defective in the inhibitory vulval pathway.
The let-23 primary structure resembles that of the mammalian epidermal growth factor receptor (EGFR). The let-23 protein possesses putative ligand binding,transmembrane, and tyrosine kinase domains, as well as cysteine-rich regions, all with the characteristics of the EGFR family. Like let-23, mammalian EGFR is multifunctional, encodes tissuespecific functions, and functions in stimulatory and inhibitory pathways. let-23 may be the receptor in the vulva for the anchor-cellinductive signal. Furthermore, genetic data indicate let-23 acts upstream of the let-60 ras gene, supporting mammalian studies that suggest a link between EGFR and ras.
To investigate how EGFR primary structure relates to function, mutations in eight let-23 alleles have beensequenced. Five null alleles alter sequences in both the kinase and the extracellular domains. These alterations suggest that let-23 has kinase activity and that the extra cysteine domain found only in invertebrate EGFRs is important. A strong hypomorphic allele mutates one of the conserved extracellular cysteines close to the ligand binding domain. A tissue-specific allele mutates anintronlexon boundary in the C-terminus. This mutation suggests that the C-terminus can provide tissue-specific information. Finally, a hypomorphic allele that is defective in the let•23 inhibitory vulval pathway alters a different intron/exon boundary in the C-terminus. This mutation results in numerous, unexpected transcripts. Models are suggested to account for the behavior of this allele.
