Homologs of vertebrate growth factors in Drosophila melanogaster and other invertebrates.
The molecular characterization of a number of loci that control developmental processes in invertebrates has revealed that a subset of these genes encode products that are homologous to vertebrate growth factors. Genetic analyses of the autonomy of action and molecular analysis of the patterns of expression of these genes have demonstrated that products of some of these loci (e.g., the EGF homologs, Notch, Delta, lin-12, and glp-1) appear to act in a cell-autonomous manner, while the products of other such loci (e.g., the TGF-beta homolog decapentaplegic and the murine int-1 homolog wingless) act in a nonautonomous manner. Studies of a number of invertebrate EGF homologs, including Notch, Delta, lin-12, and glp-1, for which we are beginning to achieve some reasonable understanding, reveal three common themes. First, each of these loci had been implicated in the determination of cell fates. The products of these loci appear to act at the level of single cells (i.e., they are required for the local choice between alternative determined states). The action of each of these loci within the context of determinative processes is clearly pleiotropic; mutations in each of these genes are correlated with multiple developmental defects. Second, the preponderance of evidence indicates that products of each of these loci function in a cell-autonomous manner during development. This shared character implies that these loci do not encode precursors of EGF-like molecules that act, in turn, as diffusible effectors in determinative decisions. It appears, rather, that these molecules function in association with the membranes of the cells in which they are produced and may constitute components of a class of receptors required for sensing diverse cues that specify particular cell fates during development. Third, we propose that EGF-like sequences found within each of these products function as protein-protein contact motifs that are essential for intermolecular interactions that involve membrane-bound molecules and are central to determinative decisions during development. Assignment of such a function to these sequences is consistent with recent findings indicating that EGF-homologous sequences found in urokinase (Apella et al., 1987) and blood coagulation factor IX (Rees et al., 1988) constitute sites that are required for binding to appropriate interacting proteins and are distinct from the respective "active" sites of each molecule. Within the context of this proposal, products of the EGF-homologous invertebrate genes noted above would participate in the transfer of information required for the specification of cell fate from the extracellular compartment to the cell interior.(ABSTRACT TRUNCATED AT 400 WORDS)