Worm Breeder's Gazette 15(1): 53 (October 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-3 Structure / Function Studies

Cathy Savage-Dunn1, Huang Wang2, Richard W. Padgett2

1 Department of Biology, Queens College, CUNY, Flushing, NY 11367
2 Waksman Institute, Rutgers University, Piscataway, NJ 08855

     A TGF-beta signal transduction cascade controls body size and
aspects of male tail morphogenesis in C. elegans, as evidenced by the
Sma and Mab phenotypes of a TGF-beta-like ligand, dbl-1 (Yo Suzuki and
Bill Wood, personal communication), the TGF! receptors, daf-4 (Estevez
et al., Nature, 1993; Savage et al., 1996) and sma-6 (Srikant Krishna
and Rick Padgett, unpublished data), and the TGF-beta signal
transducers sma-2, sma-3, and sma-4 (Savage et al., PNAS, 1996). sma-2,
sma-3, and sma-4 are members of a conserved family of TGF-beta signal
transducers, the Smads, that function downstream of TGF-beta receptors.
Biochemical studies on the vertebrate Smads has led to a model in which
Smad proteins are activated by phosphorylation by TGF-beta receptors
allowing them to translocate to the nucleus and act as transcriptional
     Smad proteins feature two conserved domains of ~100 amino acids,
separated by a more divergent proline-rich linker. Although they are
highly conserved, the Smads contain no recognizable structural motifs.
Much work remains to be done to understand what structural features of
the proteins are required for their activities. To begin to dissect the
functional domains of these proteins, we are analyzing the structure
and function of the sma-3 gene through the analysis of 10 sma-3 mutants
identified in genetic screens. sma-3 mutants fall into two phenotypic
classes, 3 weak alleles and 7 strong alleles, based on the penetrance
of the male tail sensory ray fusions (Mab phenotype). In general, the
severity of the Sma phenotype is also proportional to the penetrance of
the Mab phenotype. The following table gives the percent frequency that
a given ray is fused with another ray.

Allele    Ray 4       Ray 5   Ray 6   Ray 7   Ray 8   Ray 9   N
wk20         0       0         6       6       0       0     16
wk23          0       0       0       0       0       0     8
e958*         5       5       8       8       ND      ND    118
wk21          14      14      86      86      14      14    7
wk24          10      10      70      70      20      20    10
wk27          12      12      65      65      0       0     17
wk28          9       18      91      91      27      27    22
wk30          0       0       100     100     20      20    5
e491*         28      38      34      37      ND      ND    67
e637*         11      11      74      74      ND      ND    119
*Savage et al., PNAS, 1996.

     We are sequencing the molecular lesions in these mutant alleles.
Two of the strong alleles (e637 and wk27) are nonsense mutations in the
linker and in the N-terminal domain, respectively; three others (e491,
wk21 and wk28) are missense mutations in conserved amino acids in the
C-terminal domain. The C-terminal domain is also the site of most of
the identified mutations in other Smad genes. Interestingly, the single
weak allele that we have sequenced (wk20) has a missense mutation in a
conserved amino acid in the N-terminal domain. This result suggests
that the N-terminal domain may not be completely essential for the
function of the Smad protein.