R-Smad competition controls activin receptor output in Drosophila

PLoS One. 2012;7(5):e36548. doi: 10.1371/journal.pone.0036548. Epub 2012 May 1.

Abstract

Animals use TGF-β superfamily signal transduction pathways during development and tissue maintenance. The superfamily has traditionally been divided into TGF-β/Activin and BMP branches based on relationships between ligands, receptors, and R-Smads. Several previous reports have shown that, in cell culture systems, "BMP-specific" Smads can be phosphorylated in response to TGF-β/Activin pathway activation. Using Drosophila cell culture as well as in vivo assays, we find that Baboon, the Drosophila TGF-β/Activin-specific Type I receptor, can phosphorylate Mad, the BMP-specific R-Smad, in addition to its normal substrate, dSmad2. The Baboon-Mad activation appears direct because it occurs in the absence of canonical BMP Type I receptors. Wing phenotypes generated by Baboon gain-of-function require Mad, and are partially suppressed by over-expression of dSmad2. In the larval wing disc, activated Baboon cell-autonomously causes C-terminal Mad phosphorylation, but only when endogenous dSmad2 protein is depleted. The Baboon-Mad relationship is thus controlled by dSmad2 levels. Elevated P-Mad is seen in several tissues of dSmad2 protein-null mutant larvae, and these levels are normalized in dSmad2; baboon double mutants, indicating that the cross-talk reaction and Smad competition occur with endogenous levels of signaling components in vivo. In addition, we find that high levels of Activin signaling cause substantial turnover in dSmad2 protein, providing a potential cross-pathway signal-switching mechanism. We propose that the dual activity of TGF-β/Activin receptors is an ancient feature, and we discuss several ways this activity can modulate TGF-β signaling output.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Activin Receptors / genetics
  • Activin Receptors / metabolism*
  • Animals
  • Blotting, Western
  • Cell Line
  • DNA-Binding Proteins / genetics
  • DNA-Binding Proteins / metabolism*
  • Drosophila Proteins / genetics
  • Drosophila Proteins / metabolism*
  • Drosophila melanogaster / cytology
  • Drosophila melanogaster / genetics
  • Drosophila melanogaster / metabolism*
  • Female
  • Larva / growth & development
  • Larva / metabolism
  • Mutation
  • Phosphorylation
  • Protein-Serine-Threonine Kinases / genetics
  • Protein-Serine-Threonine Kinases / metabolism
  • RNA Interference
  • Receptor Cross-Talk
  • Receptors, Cell Surface / genetics
  • Receptors, Cell Surface / metabolism
  • Receptors, Transforming Growth Factor beta / genetics
  • Receptors, Transforming Growth Factor beta / metabolism
  • Signal Transduction
  • Smad Proteins, Receptor-Regulated
  • Smad2 Protein / genetics
  • Smad2 Protein / metabolism*
  • Transcription Factors / genetics
  • Transcription Factors / metabolism*
  • Transforming Growth Factor beta / genetics
  • Transforming Growth Factor beta / metabolism
  • Wings, Animal / growth & development
  • Wings, Animal / metabolism

Substances

  • DNA-Binding Proteins
  • Drosophila Proteins
  • MAD protein, Drosophila
  • Receptors, Cell Surface
  • Receptors, Transforming Growth Factor beta
  • Smad Proteins, Receptor-Regulated
  • Smad2 Protein
  • Smox protein, Drosophila
  • Transcription Factors
  • Transforming Growth Factor beta
  • sara protein, Drosophila
  • sax protein, Drosophila
  • tkv protein, Drosophila
  • Protein-Serine-Threonine Kinases
  • Activin Receptors
  • Babo protein, Drosophila