CELL SIGNALLING IN C ELEGANS

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The long term goal of the proposed research is to understand how cell signaling controls cell growth and differentiation, using vulval induction in C. elegans as a model system. Many signaling genes that control vulval induction have been genetically and molecularly identified. Some are homologs of genes that constitute the receptor tyrosine kinase pathway in mammals and Drosophila, including let-23 (a receptor tyrosine kinase gene), let-60 (a ras homolog) and mpk-1 (a MAP kinase homolog). Other vulval signaling genes (such as lin-2, lin-7 and lin-10) are likely to extend this highly-conserved signaling pathway because they may identify new signaling steps, since they are not similar to genes that have previously been identified in this type of signaling pathway. Protein motifs found in LIN-2 and LIN-7 suggest several possible explanations about how these proteins interact with the receptor tyrosine kinase pathway. These proteins are similar to a class of tight junction proteins in mammals and Drosophila, suggesting that LIN-2 and LIN-7 (possible LIN-10 as well) may be constituents of the nematode tight junction, and that their function is to localize the LET-23 receptor to the basal surface of polarized vulval precursor cells. In this model, this asymmetric distribution of the LET-23 receptor would be essential for its activation. LIN-2 also contains a protein domain that is similar to an enzyme involved in guanine nucleotide metabolism, suggesting an alternative model involving an interaction between LIN-2 and the GTPase activity of Ras. Three approaches will be used to determine the role of these new signaling genes in vulval induction. First, each model makes specific predictions regarding the functional importance of the various lIN-2 protein domains and predicted enzymatic activities; these predictions will be tested. Second, the interactions between LIN-2, LIN-7 and LIN-10 will be defined by determining whether these proteins are localized to the same region within the cell and whether they physically interact with each other. Third, the receptor localization model will be tested by determining whether the LET- 23 receptor is concentrated on the basal surface of vulval precursor cells, and whether this polarized distribution requires LIN-2, LIN-7 and LIN-10. Data supporting this model will further our understanding of the important but poorly understood relationship between cell polarity and cell signaling. The proposed investigation of new cellular functions involved in a highly- conserved receptor tyrosine kinase signaling pathway is relevant to understanding tumor formation, since constitutive activation of similar pathways in mammalian cells is oncogenic.