Worm Breeder's Gazette 14(5): 58 (February 1, 1997)

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.

sma-6 is Involved in Body Size Determination and Tail Morphogenesis

Srikant Krishna, Daniel Collins, Richard W. Padgett

Waksman Institute, Rutgers University Hoes Lane, Piscataway, NJ 08855

In C. elegans, two TGF-beta-like signaling systems have been identified
which utilize a common type II receptor, daf-4. The phenotypes of daf-4
include constitutive dauer formation, small body size, and male tail
abnormalities. daf-1 has previously been shown to encode a TGF-beta
superfamily type I serine-threonine receptor kinase, and mutations in
this gene effect constitutive dauering. Based on our work in
characterizing the downstream SMAD components sma-[2,3,4], we had
postulated the existence of another type I receptor which was involved
in those phenotypes shared by these SMADs and daf-4, namely body size
and tail development.

We had therefore cloned a novel type I receptor using a molecular
approach, and have shown that it is encoded by the gene sma-6. We are
now interested in analyzing the developmental role of sma-6 in more
detail. We have obtained novel alleles of sma-6, wk7, wk8, and wk9 in
screens for small mutants, and we also have performed a
non-complementation screen using the canonical e1482 allele.

The e1482 allele contains a mutation in the extracellular region,
90A->V. This modification is most likely not severe as deduced from the
amino acid replacement, and this reasoning is supported by the
observation that e1482 animals are not as small as other mutants in the
pathway. The wk7 allele, however, is the result of a mutation which
changes residue 72 in the extracellular region into a premature
termination. Because the expressed protein lacks most of the
extracellular domain, and does not extend into the transmembrane and
kinase domains, we conclude that wk7 represents a null allele of sma-6.
Furthermore, the wk7 phenotype displays a body size that is consistent
with those exhibited by mutations in sma-[2,3,4]. In addition,
hemizygous wk7 animals do not show an observable difference in body
size.  wk7 does not exhibit antimorphic characteristics. We are
currently determining the molecular change in the other sma-6 alleles,
including a Tc-1 induced transformation (wk9).

Because the phenotypes of the daf-4 and the sma-[2,3,4] genes, when
mutant, also result in characteristic defects in tail development, we
have analyzed the male tails of the sma-6 alleles.  It can be
hypothesized that another type I receptor is involved in this process,
but our results argue against this. Animals homozygous for the e1482
allele do not exhibit the abnormalities, specifically crumpled spicules
and ray fusions, present in other genes in the small pathway. However,
the finding that these deformities are present in wk7,wk8, and wk9 males
confirms the notion of daf 4, sma-2, sma-3, sma-4, and sma-6 functioning
in a common pathway. 

Expression studies using a translation fusion of a sma-6 promoter and
coding segment with lacZ are being examined. These results will be
compared with antibodies we generated against the extracellular region
of sma-6 to extend this result.

These findings demonstrate the potential for disparate TGF-beta
signaling pathways to intersect at a common type II receptor, and
investigate the role of this novel receptor in C. elegans development.
It will be meaningful to explore the applicability of this work to other
TGF-beta-like systems.