Worm Breeder's Gazette 14(2): 42 (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.

The Giant Dwarfin Family

Pradeep Das, Richard W. Padgett

Waksman Institute, Rutgers University, Piscataway, NJ-08855

TGF-b signalling pathways are important in the regulation of cell growth
and differentiation, and several aspects of development in various
organisms, yet the components of these pathways are relatively unknown.
In trying to elucidate the signalling pathway of the dpp/BMP family of
TGF-b-like molecules, we are studying three homologous C. elegans genes,
sma-2, sma-3 and sma-4 (Savage et al., PNAS 1996), that are believed to
be in the pathway, acting downstream of the type II serine-threonine
kinase receptor daf-4. These genes give some mutant phenotypes  similar
to those of daf-4 (small body size, male-tail abnormalities), and mosaic
analysis indicates that they are required cell-autonomously and in the
same cell as daf-4. Sma-2, Sma-3, and Sma-4, along with the Drosophila
Mad gene product (Sekelsky et al., 1995), are members of the dwarfin
family: a novel set of proteins characterised by two highly conserved
regions with a proline-rich region in between. The functions of these
regions are not yet known, but we have shown that the smas act in the
daf-4 pathway to define body size.

Members of the Dwarfin Family
In trying to determine whether these genes represent a pathway conserved
in vertebrates, we used degenerate PCR with primers designed against
regions of homology conserved between the invertebrate members (Das et
al.,WM95, pg.176). We have successfully isolated three murine and three
human homologues and one Xenopus homologue. Our analysis of the dwarfin
family predicted the existence of multiple members in other organisms,
including Drosophila, and, in looking intensively to find these, we have
thus far isolated demented, a sma-4-like gene.

Since daf-4 mutations show the daf-c and egl phenotypes in addition to
the sma and mab phenotypes of the sma genes, we wondered whether there
might be any dwarfins acting to transduce the other signals from DAF-4.
Recently, two other groups have reported that other genes, known to act
in the dauer pathway, are also dwarfins. Furthermore, a search of the
shotgun sequences from the C. elegans genome sequencing project has
revealed the presence of at least two other members; one maps close to
daf-14 and the other to XR (cosmid # F52G3). The latter, tentatively
named dwa-1 (for dwarfin-1) does not correspond to any known dauer or
small locus. We are in the process of identifying mutations in it. It
will be interesting to see if this gene also acts downstream of daf-4,
causing either the sma and mab phenotypes, or the daf and egl

Functional Characterisation of the Dwarfins
We are exploring several methods to outline the possible functions of
the dwarfins. We are carrying out genetic screens to determine other
phenotypes and other members of the pathway (Savage et al., this

Given the great degree of similarity between these proteins, it is
tempting to speculate that they may actually interact with each other in
a linear order or function as a complex. To differentiate between the
two models, we are using the Brent yeast two-hybrid interaction trap
system to look for interactions within the SMAs and between the SMAs and

In addition to the experiments on sma-2, -3, and -4 outlined above, we
are characterising the vertebrate dwarfins in several ways. We are
collaborating on doing in-situ hybridisations against the mouse embryo
with the murine dwarfins to look for overlaps in patterns of expression
with other components of TGF-b-like pathways, for eg. the ligands BMP-2
and -4 and the receptor BRK-1. We are also collaborating on performing
biochemical assays using mammalian cell lines. Since vertebrates have
several TGF-b-like pathways, we are working with the vertebrate dwarfins
to explore the important issue of whether the dwarfins act specifically
in the dpp/BMP pathway or whether they are a more ubiquitous mechanism
for transducing TGF-b-like signals.