Department of Cell and Developmental Biology, University of Michigan, Ann Arbor, MI, USA
(e-mail: hortsch{at}umich.edu)
Two recent articles published in Development describe the role of
the Drosophila Echinoid (Ed) protein in R8 photoreceptor cell
development and in Egfr signalling (Rawlins
et al., 2003; Spencer and
Cagan, 2003
). Bai et al. first described Ed as an Ig-domain
membrane protein and as an antagonist of Egfr signalling in the developing
Drosophila eye (Bai et al.,
2001
). Both recent publications, which were highlighted in the `In
this issue' feature, speculate that Ed might be an L1-type cell adhesion
molecule (CAM). Similar to L1-type proteins, Ed is a homophilic CAM
(Islam et al., 2003
), and both
proteins are members of the Ig-domain superfamily. However, Ed is not part of
the L1 family and has a different protein domain structure (usually six Ig-
and five FNIII-protein domains for L1-type proteins versus seven plus two for
Ed). Furthermore, Ed, as well as its Drosophila paralogue Fred
(Chandra et al., 2003
), has a
strikingly different cytoplasmic domain and lacks the landmark ankyrin binding
site of L1-type CAMs. The Drosophila genome contains only one L1-type
gene, called Neuroglian (Nrg)
(Hortsch, 2000
). Interestingly,
the Nrg protein is a heterophilic ligand of Ed and triggers the Egfr
antagonist activity of Ed in a synergistic manner
(Islam et al., 2003
).
REFERENCES
Bai, J., Chiu, W., Wang, J., Tzeng, T., Perrimon, N. and Hsu,
J. (2001). The cell adhesion molecule Echinoid defines a new
pathway that antagonizes the Drosophila EGF receptor signaling
pathway. Development
128,591
-601.
Chandra, S., Ahmed, A. and Vaessin, H. (2003). The Drosophila IgC2 domain protein friend-of-echinoid, a paralogue of echinoid, limits the number of sensory organ precursors in the wing disc and interacts with the Notch signaling pathway. Dev. Biol. 256,302 -316.[CrossRef][Medline]
Hortsch, M. (2000). Structural and functional evolution of the L1-family: are four adhesion molecules better than one? Mol. Cell. Neurosci. 15,1 -10.[CrossRef][Medline]
Islam, R., Wei, S.-Y., Chiu, W.-H., Hortsch, M. and Hsu,
J.-C. (2003). Neuroglian activates Echinoid to antagonize the
Drosophila EGF receptor signaling pathway.
Development 130,2051
-2059.
Rawlins, E. L., White, N. M. and Jarman, A. P.
(2003). Echinoid limits R8 photoreceptor specification by
inhibiting inappropriate EGF receptor signalling within R8 equivalence groups.
Development 130,3715
-3724.
Spencer, S. A. and Cagan, R. L. (2003).
Echinoid is essential for regulation of Egfr signaling and R8 formation during
Drosophila eye development. Development
130,3725
-3733.