C-terminal mutants of C. elegans Smads reveal tissue-specific requirements for protein activation by TGF-beta signaling.
TGF-beta signaling in the nematode Caenorhabditis elegans plays multiple roles in the development of the animal. The Sma/Mab pathway controls body size, male tail sensory ray identity and spicule formation. Three Smad genes, sma-2, sma-3 and sma-4, are all required for signal transduction, suggesting that the functional complex could be a heterotrimer. Because the C termini of Smads play important roles in receptor-mediated activation and heteromeric complex formation, we generated C-terminal mutations in the C. elegans Smad genes and tested their activities in vivo in each of their distinct developmental roles. We show that pseudophosphorylated SMA-3 is dominant negative in body size, but functional in sensory ray and spicule development. Somewhat differently, pseudophosphorylated SMA-2 is active in any tissue. The C-terminal mutants of SMA-4 function like wild type, suggesting that the SMA-4 C terminus is dispensable. Using a combination of different C-terminal mutations in SMA-2 and SMA-3, we found a complex set of requirements for Smad-phosphorylation state that are specific to each outcome. Finally, we detected a physical interaction of SMA-3 with the forkhead transcription factor LIN-31, which is enhanced by SMA-3 pseudophosphorylation and reduced in an unphosphorylatable mutant. We conclude that the tissue-specific requirements for Smad phosphorylation may result, in part, from the need to interact with tissue-specific transcription co-factors that have different affinities for phosphorylated and unphosphorylated Smad protein.
Department of Biology, Queens College, and Graduate School and University Center, City University of New York, 65-30 Kissena Boulevard, Flushing, NY 11367, USA.
Jianjun
J
Wang
William A
WA
Mohler
Cathy
C
Savage-Dunn
D000818:Animals; D001483:Base Sequence; D049628:Body Size; D017173:Caenorhabditis elegans / Q000196:embryology / Q000235:genetics*; D029742:Caenorhabditis elegans Proteins / Q000235:genetics*; D017931:DNA Primers; D004268:DNA-Binding Proteins / Q000235:genetics*; D008297:Male; D009024:Morphogenesis; D016296:Mutagenesis; D010766:Phosphorylation; D015183:Restriction Mapping; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology*; D051785:Smad Proteins; D013623:Tail / Q000196:embryology; D015534:Trans-Activators / Q000235:genetics*; D014157:Transcription Factors / Q000235:genetics; D016212:Transforming Growth Factor beta / Q000494:pharmacology*
D029742:Caenorhabditis elegans Proteins; D017931:DNA Primers; D004268:DNA-Binding Proteins; D051785:Smad Proteins; D015534:Trans-Activators; D014157:Transcription Factors; D016212:Transforming Growth Factor beta; C097873:sma-2 protein, C elegans; C097874:sma-3 protein, C elegans; C097875:sma-4 protein, C elegans