The
let-23 gene encodes a member of the EGF receptor tyrosine kinase family and is required for the development of the hermaphrodite vulva and four other tissues. During vulval induction,
let-23, which may be the receptor for the anchor cell inductive signal, is required for two opposing pathways: (1 ) stimulation of precursor cells to execute vulval rather than hypodermal fates, and (2) inhibition of the inductive process to prevent too much induction from occuring. Also, the
let-23 gene functions differently in different tissues since it is possible to mutate independently
let-23 function in the different tissues. Consistent with these genetic results, members of the EGF receptor family have been shown to behave differently in different tissues and to be subject to negative regulation. Mammalian EGF receptor comprises several structural domains. Extracellulary, there is a ligand binding domain surrounded by two cysteine-rich regions thought to form a 'pocket.' The cytoplasmic portion of the protein contains the tyrosine kinase domain, autophosphorylation sites, and a domain required for receptor internalization. To relate our genetic characterization of
let-23 to what is known of EGF receptor structure and to broaden our knowledge of both
let-23 and EGF receptor function, we are using hydroxylamine mismatch detection to localize and sequence the mutations associated with 22
let-23 alleles. Fourteen alleles behave like nulls, two behave like hypomorphs, four exhibit tissue-specific defects, and two are defective in the inhibitory vulval pathway. The sequence of these alleles might suggest: (1 ) residues critical for receptor function, ( 2) residues responsible for tissue specificity, and (3) residues involved in negative regulation. For example, two null mutations alter conserved residues in the tyrosine kinase domain, suggesting that these residues are functionally important. Another mutation, nlO45, is located in an exon/intron boundary and potentially deletes C-terminal negative regulatory elements, consistent with the observation that this allele results in defects in the inhibitory vulval pathway.