The accompanying figure shows sequence alterations associated with eight
lin-12(d) alleles. All eight
lin-12(d) mutations examined are missense alterations of the protein coding region and are therefore likely to be modifying protein activity or increasing protein stability. Thus, the fact that
lin-12(d) mutations behave like genetic hypermorphs (Greenwald et al., Cell 34: 435 -444,1983) would be consistent with two possibilities: either the
lin-12(+) allele contributes a basal level of
lin-12 activity that can augment
lin-12(d) activity, or the
lin-12(d) protein transactivates the
lin-12(+) protein. We favor the second possibility, in light of genetic studies suggesting that
lin-12 interacts with itself (Geraldine Seydoux and I. G., unpublished observations) and biochemical evidence that Notch can form dimers (Kidd et al., Genes & Dev. 3:1113-1129,1989). Our current favorite working hypothesis is that intercellular signals promote self-association of
lin-12, thereby activating a signal transduction mechanism. This model is plausible in view of what is known about biochemically-defined receptors: in many cases, the association state of receptors in the membrane controls their activity, and association state is in turn controlled by ligand-binding ( reviewed in Andersen, Nature 337:12,1989). The
lin-12(d) mutations could lead to activation in the absence of signals necessary to activate
lin12(+) protein. Consistent with this explanation is the observation that if all gonadal cells except for Z1.ppp or Z4.aaa are ablated in a
lin-12(
n137) hermaphrodite, it becomes a VU; in contrast, in wild type, it would become an AC, presumably because it lacks a necessary signal (Seydoux and Greenwald, Cell 57:1237-1245,1989). [See Figure 1] Predicted amino acid changes (residue number: change) deduced from sequence analysis of different
lin-12(d) alleles are shown on a schematic depiction of the
lin-12 product (Yochem et al., Nature 335: 547-550,1988). EGF = EGF-like motifs; T = 'T + Y' encoded motif; LNR =
lin12/Notch repeated motif;
cdc10 =
cdc10/SWI6 motif; C = position of a pair of cysteines that is conserved in
lin-12, h. Kidd et al. (1989) propose that these cysteines mediate the dimerization of Notch, and it is curious that they are conserved in a region of the
lin-12 protein that shows no overt sequence similarity to Notch.