Genetic screens have previously implicated a role for the C. elegans pRb tumor suppressor ortholog, LIN-35, in pharyngeal development and morphogenesis (1, 2). We find that LIN-35, together with UBC-18/UBCH7-ARI-1/ARIH1, a conserved E2-E3 ubiquitin-ligase complex, cooperate to repress the expression of the Zn-finger protein, SUP-35/ZTF-21 (3-5). Whereas LIN-35 acts in conjunction with the conserved E2F pathway members EFL-1 and HCF-1 to regulate
sup-35 transcription, UBC-18-ARI-1 appears to regulate SUP-35 at the level of protein stability. SUP-35 in turn acts to repress the expression of PHA-1, a cytoplasmic protein of unknown function that is required for embryonic and pharyngeal development (6-8). Thus, in
lin-35;
ubc-18 double mutants, PHA-1 expression is strongly reduced due to increased levels of the SUP-35 transcriptional repressor. Furthermore, we find that inhibition of PHA-1 expression by SUP-35 requires two additional factors, SUP-37/ZTF-12, a Zn-finger protein, and SUP-36/F38A5.7, a novel protein. Genetic epistasis and expression data suggest that SUP-35-37 are likely to act a common step, possibly within a single complex, in order to repress
pha-1 transcription. Additionally, SUP-37 carries out an essential role in larval pharyngeal pumping that is separable from its role in PHA-1 regulation. Our findings also suggest a potential role for the microtuble cytoskeleton in the activity of SUP-35-SUP-37 given that (1) SUP-36 can physically associate with PTL-1, a member of the tau family of microtuble-binding proteins, (2) SUP-35 contains an RMD domain that places it within a conserved family of regulators of microtubule dynamics, and (3)
pha-1 mutants show strong synthetic lethality with
tbb-1, which encodes for a homolog of beta-tubulin. Finally, a genome-wide RNAi suppressor screen has identified a number of additional factors that contribute to the regulation of this developmental network. REFERENCES: 1.D. S. Fay, S. Keenan, M. Han, Genes Dev 16, 503 (2002). 2.D. S. Fay, E. Large, M. Han, M. Darland, Development 130, 3319 (2003). 3. K. Mani, D. S. Fay, PLoS genetics 5,
e1000510 (2009). 4. X. Qiu, D. S. Fay, Dev Biol 291, 239 (2006). 5. H. Schnabel, G. Bauer, R. Schnabel, Genetics 129, 69 (1991). 6. D. S. Fay et al., Dev Biol 271, 11 (2004). 7. M. Granato, H. Schnabel, R. Schnabel, Development 120, 3005 (1994). 8. H. Schnabel, R. Schnabel, Science 250, 686 (1990).