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.

A tre-1 in a Forest

Srikant Krishna, Richard W. Padgett

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

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.