Worm Breeder's Gazette 14(2): 45 (February 1, 1996)
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
Waksman Institute, Rutgers University, Piscataway, NJ 08855
The TGF-b-like signalling response involves a crucial ligand-receptor interaction for transduction to occur. The TGF-b-like ligands share a highly conserved C-terminus and can bind to receptors in a dimeric fashion, linked by a cysteine bridge. Several types of receptors that are able to bind to TGF-b superfamily members in vitro have been reported. Of these, the type I and II receptors contain a serine/threonine kinase intracellular domain which is essential for proper signalling to occur. It has been demonstrated in Drosophila and in vertebrate systems that multiple type I receptors can interact with a single type II receptor. This presumably determines part of the specificity in the signalling of TGF-b superfamily members. For example, in the fly, the products of sax and tkv (type I receptors) interact with the punt (type II receptor) gene product and can send slightly different dpp signals. The type I and II receptors share significant aligment homology, but differ in several regions. They each have a poorly conserved extracellular domain, a variable hydrophobic stretch constituting a transmembrane domain, followed by the kinase domain. In addition, it is thought that the GS-box motif, which is the hallmark of a type I receptor, is required in the proper interaction of a type I and type II receptor upon ligand binding. This would then be followed by the subsequent phosphorylation of the type I receptor by the type II. It is then postulated that the activated receptor system further transduces the signal. In the worm, the dauer and small pathways are TGF-b-like systems which have been identified. In the dauering pathway, the products of daf-1, daf-4 and daf-7 serve to transduce the signal. The dowstream targets and components of this dual-receptor system are not well understood, but putative constituents are being identified. The dwarfins comprise a group of genes which encode products that have been implicated in the TGF-b transduction pathway (Savage et al., PNAS 1996; Das et al., this Gazette; Savage et al., WBG 1995) in both Drosophila (Sekelsky et al., Genetics 139: 1347-1358, 1995) and the nematode, mutations in which yield the small phenotype. In the worm, the sma-2, sma-3, and sma-4 genes are dwarfins that have been characterized by our group. The mutant phenotypes of these genes and are a subset of those exhibited by daf-4. However, the sma phenotypes are neither observed in daf-1 nor in daf-7 mutants. Thus, daf-4 intersects the dauering and small pathways. Since daf-1 does not exhibit the small phenotype, we therefore hypothesize that there should be another type I receptor, that when mutant, results in small animals, since daf-1 does not exhibit this phenotype. Based on this logic, we had postulated the existence of additional type I receptors which are involved in transducing a TGF-b-like signal to the sma gene products. Additionally, in other systems such as Drosophila, multiple type I receptors exist, including sax (Xie et al., Science 1994) and tkv. Motivated by this reasoning, we sought novel receptors in the worm that are similar to sax or tkv. We performed RT-PCR upon N2 total mRNA using degenerate primers corresponding to regions VI and VIII of the kinase domain and have identified the tre-1 gene (TGF-b-like receptor) as a type I receptor in C. elegans. The cDNA for tre-1 revealed that it shows excellent homology to the type I receptors found in other systems, particularly with sax and tkv, and that it is considerably more representative of this class than daf-1. We are currently seeking mutations in tre-1 to assess the mutant phenotype. Also, biochemical binding assays are possible with the TIG-1 and TIG-2 putative ligands (Krishna et al., this Gazette). Using the dwarfins that have been isolated in our laboratory, it will also then be possible to test for specific interactions with tre-1. Additional signal specificity may occur with different combinations of ligands that interact with a putative TRE-1/DAF-4 complex. Continued efforts to identify and characterize other possible receptor candidates will augment our knowledge of the TGF-b signalling mechanism.