Based on similarities in phenotypes and genetic interactions, five Daf-c (dauer constitutive) genes,
daf-1, -4, -7, -8 and -14 are thought to have related functions. Molecular identities of four of these genes have been reported previously by Don Riddle's group.
daf-7 encodes a homolog of TGF-beta(1),
daf-1 and
daf-4 encode homologs of TGF-beta receptors(2,3), and
daf-8 encodes a homolog of Drosophila gene Mad (Mothers against dpp)(4). We cloned
daf-14 in order to understand its function in the pathway.
daf-14 is rescued by the cosmid F01G10, recently sequenced by the genome sequencing project. From the sequence, we identified a candidate gene based on homology, which was confirmed to be
daf-14 by sequencing mutant alleles.
daf-14 is a member of the recently described gene family(5) that includes Mad from Drosophila(6) and the C. elegans genes
sma-2,
sma-3,
sma-4(5) and
daf-8(4). All of these genes are implicated in TGF-beta related signal transduction, suggesting they play a conserved role. Known members of this gene family contain two conserved regions, DH1 and DH2(5). Genefinder analysis and sequence alignment of the
daf-14 genomic DNA predicts a protein with strong similarity to the DH2 region but without a DH1-like domain. Also, no homology to DH1 was detected in a search of genomic sequence upstream of
daf-14 DH2 region. This suggests that
daf-14 is unique in not having a DH1 region. However, because the mRNA structure has not been confirmed, we do not rule out the possibility that the DH1 domain is present. All three mutations in
daf-14 disrupt highly conserved residues in the C terminal half of DH2 region. We are continuing to study the role of
daf-14 in TGF-beta signaling. We generated two integrants of
daf-14(+)-carrying transgenic arrays which probably overexpress the
daf-14 gene product. These arrays rescue the Daf-c defect of
daf-14 and
daf-8, but not
daf-1, -4 and -7. These results are consistent with many models, including epistatic interactions and functional substitution of
daf-14 for
daf-8. 1. Lim et al., WM93 abstract
p15, Lim, Ph.D. Thesis, University of Missouri, Columbia, Missouri. 2. Georgi et al., Cell 61:635-645 3. Estevez et al., Nature 365:644-649 4. Estevez and Riddle, WBG14.2
p37 5. Savage et al., PNAS USA 93 790-794 6. Sekelsky et al., Genetics 139: 1347-1358