Worm Breeder's Gazette 14(2): 44 (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.

tig-1, tig-2: Novel TGF-b Homologs in C. elegans

Srikant Krishna, Richard W. Padgett

Waksman Institute, Rutgers University, Piscataway, NJ 08855

The TGF-b superfamily comprises a group of secreted proteins which are
involved in fundamental biological processes including, but not limited
to, axis specification, mesoderm induction, osteogenic differentiation,
tissue regeneration, and neoplastic growth.  There is significant
primary structure homology shared among all the members, and the
superfamily can be further partitioned into groups based upon homology
and function.  These families include the dpp/BMP group, the
activin/inhibin family, the TGF-b subset, the MIS family, and others. 
Though there has been intense interest in recent years to determine the
pathways by which TGF-b signalling occurs, the mechanism by which the
signal is transduced from a TGF-b superfamily members is still
relatively poorly understood.  

Membrane receptors which specifically bind with TGF-b-like ligands have
been identified and are being characterized in several organisms.  A
subset of these receptors contain a serine-threonine kinase domain, and
can be categorized into two classes, type I and type II, which differ
considerably with each other in their non-kinase regions.   A general
model for the transduction of the TGF-b-like signal involves an initial
proteolytic modification of the ligand followed by binding of mature
cysteine-bridged dimeric form with the type II receptor.  Upon binding,
interaction between the two types of receptors can occur, and mutual
phosphorylation is implicated in receptor activation.  In this activated
state, the receptor kinases presumably are then capable of transducing
the signal through the phosphorylation of other targets.  This cascade
would eventally result in biochemical changes and altered expression
patterns of specific genes.   

In our laboratory, we have been trying to elucidate of the TGF-b signal
transduction system in C. elegans.  We have identified candidate genes
at each step of the transduction process in the nematode using a variety
of methods.  Putative homologs at the metalloprotease (Finelli et al.,
this Gazette), ligand, and receptor (Krishna et al., this Gazette) 
level have been identified and are being characterized.  Additionally,
attempts to isolate downstream components have yielded the dwarfin
family of potential downstream mediators (Das et al., this Gazette),
including sma-2, sma-3, and sma-4, which have been implicated in the
pathway through both molecular and genetic observations (Savage et al.,
this Gazette). 

In C. elegans, dbl-1 (formerly ceg-1;  Yandell and Wood, WBG, 1994) and
daf-7 (pers. comm. D. Riddle) have been the previously reported TGF-b
superfamily member.  Sequence alignment analysis had revealed that dbl-1
bears considerable homology to dpp, a developmentally significant
molecule in Drosophila.  DAF-7 is a divergent ligand which has been
implicated in the dauer pathway.  However, there were several lines of
evidence that illuminated the possibility that other TGF-b-like ligands
exist in the nematode.  First, comparison with other organisms such as
Drosophila, in which multiple superfamily members have been identified
and characterized, suggests that an analogous situation may exist in the
worm.  Secondly, both DPP and BMP-4 have been shown to bind with DAF-4
in-vitro;  thus we hypothesized that in the worm, multiple
dpp/BMP-related ligands should be present.  

tig-1 and tig-2, acronyms for TGF-b-like ligand, were identified using a
combination of PCR and database searches against the sequencing project.
These genes are located on cosmids C53D6 and F39G3, respectively. 
Evolutionary trees constructed using with a variety of TGF-b superfamily
members indicate that tig-1 and tig-2 show homology to the 60A and screw
genes of Drosophila.  We are currently trying to establish a definitive
evolutionary relationship among the homologs.  In addition, mutations in
tig-1 and tig-2 are being sought which would indicate their role in
devlopment and place them into known TGF-b-like signalling pathways.