In animal development, numerous cell-cell interactions are mediated by the
GLP-1/LIN-12/NOTCH family of transmembrane receptors. These proteins functi
on in a signaling pathway that appears to be conserved from nematodes to hu
mans. We show here that the aph3 gene is a new component of the GLP-1 signa
ling pathway in the early Caenorhabditis elegans embryo, and that proteins
with sequence similarity to the APH-2 protein are found in Drosophila and v
ertebrates. During the GLP-1-mediated cell interactions in the C, elegans e
mbryo, APH-2 is associated with the cell surfaces of both the signaling, an
d the responding, blastomeres. Analysis of chimeric embryos that are compos
ed of aph-2(+) and aph-2(-) blastomeres suggests that aph-2(+) function may
be provided by either the signaling or responding blastomere.