Transmembrane proteins of the LIN-12/Notch family are found throughout the animal kingdom. LIN-12/Notch proteins appear to function as receptors that are activated by binding ligands of the Delta/Serrate/LAG-2 family. However, there is no known biochemical activity associated with the intracellular domains of LIN-12/Notch proteins. An understanding of the downstream consequences of receptor activation therefore requires the identification and characterization of the interacting components involved in
lin-12/Notch mediated cell fate decisions. One such component, named Su(H) in Drosophila and LAG-1 in C. elegans, is highly similar to the mammalian CBF1 transcription factor. We have identified and characterized two other other downstream components of the
lin-12 pathway in C. elegans,
emb-5 and
sel-10. Using part of the LIN-12 intracellular domain as bait in a yeast two-hybrid system screen, we identified two clones corresponding to
emb-5. Using existing temperature-sensitive loss-of-function
emb-5 alleles(1), we have obtained evidence that the physical interaction implied by the two-hybrid screen is functionally relevant in vivo. By constructing double mutants between mutations in
lin-12 or
glp-1 and emb- 5, and by genetic mosaic analysis, we have obtained evidence that
emb-5 functions as an apparent positive regulator of
lin-12 in the "AC/VU" decision and of
glp-1 in the "mitosis/meiosis" decision during germline development. EMB- 5 is a large acidic protein that is similar to the S. cerevisiae SPT6 protein, which has been implicated in chromatin structure and/or assembly(2). Thus, EMB-5 may coordinate chromatin changes in response to LIN-12/GLP-1 activation, influencing the expression of genes involved in execution of different developmental fates. Using suppressor analysis, we identified
sel-10 as a potential negative regulator of the
lin-12 pathway(3). Hypomorphic alleles of
sel-10 elevate the activity of
lin-12 in sensitized genetic backgrounds and on their own display phenotypes associated with
lin-12 activation at low penetrance.
sel-10 alleles suppress
lin-12(null) alleles and enhance defects caused by expression of the
lin-12 intracellular domain (which is constitutively active). Since
sel-10 does not appear to act by ectopically activating glp- 1, the simplest interpretation of the genetic data is that reducing
sel-10 activity bypasses the need for
lin-12 activity. We have localized
sel-10 to an 8.5kb genomic fragment, which recognizes a 3kb transcript on a Northern blot, and hope to have sequence information by the meeting. 1. J. Miwa, E. Schierenberg, S. Miwa, G. von Ehrenstein, Dev. Biol. 76, 160-174 (1980) 2. K. Nishiwaki, T. Sano, J. Miwa, Mol. Gen. Genet. 239, 313- 322 (1993) 3. M. Sundaram, M. Greenwald, Genetics 135, 765-783 (1993)