Previous genetic analysis established that
sup-17 facilitates
lin-12 signalling (Tax et al., Genetics, in press). With the invaluable assistance of the genome project, we have cloned
sup-17 and shown that the predicted SUP-17 protein is a member of the ADAM family of proteases (see Wolfsberg and White, Devel. Biol. 180, 389-401, 1996); in particular, SUP-17 is very similar to Drosophila kuzbanian (KUZ), which functions in Drosophila and vertebrate neurogenesis (Rooke et al., Science 273, 1227-1231, 1996; Pan and Rubin, Cell 90, 271-280, 1997) (see Figure). We have extended the genetic studies of Tax et al. (1997) by showing that
sup-17 does not act downstream of
lin-12. Reduction or elimination of
sup-17 activity suppresses the effects of
lin-12(d) mutations (see Tax et al., 1997), an allelic series of hypermorphs that result from point mutations in the extracellular domain. In contrast, we have found that reducing
sup-17 activity cannot suppress mutant phenotypes associated with
lin-12(intra). The
lin-12(intra) transgene expresses the intact intracellular domain under the control of
lin-12 regulatory sequences and behaves like a constitutively active receptor. It is incompletely penetrant for its effects on the AC/VU decision and VPC specification, and hence does not elevate
lin-12 activity as much as the stronger
lin-12(d) alleles. Furthermore, we have found that
sup-17 can enhance the AC/VU decision defect of a partial loss-of-function
lin-12 allele, implying that it does not require a
lin-12(d) lesion to influence
lin-12 activity. Since
sup-17 can suppress
lin-12(d) mutations but cannot suppress
lin-12(intra), we can infer that
sup-17 does not act downstream of the activated receptor. We have also found that the interaction between
sup-17!and
lin-12 occurs within the same cell. By using a laser microbeam to isolate Z1.ppp or a vulval precursor cell from its normal source of signal, we showed that
sup-17 mutations reduce the cell intrinsic level of constitutive
lin-12 activity in
lin-12(d) mutants. This result suggests that
sup-17 acts cell autonomously and that its action is not dependent on ligand-binding. The genetic analysis of the relationship between
sup-17 and
lin-12 in C. elegans complements and extends the results from genetic studies of the relationship between the Drosophila kuz and Notch genes (Pan and Rubin, 1997). SUP-17/KUZ may function in the proteolysis of LIN-12/NOTCH proteins. There is evidence for a cleavage event in the extracellular domain of C. elegans, Drosophila, and human NOTCH proteins (Christensen et al., Development 122, 1373-1383, 1994; Blaumueller et al., Cell 90, 281-291, 1997; Pan and Rubin, 1997). Furthermore, KUZ is required in vivo for the proteolytic processing of NOTCH into two fragments consistent with a cleavage event in the extracellular domain (Pan and Rubin, 1997). Further work in several systems will be directed towards understanding how proteolysis influences LIN-12/Notch activity. Special thanks to Frans Tax, Jim Thomas, Chip Ferguson and Bob Horvitz.