Mad (Mothers against dpp; CG12399; P42003): manual literature synthesis

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Identity and family

• Mad is the founding member of the SMAD family: the Drosophila BMP-pathway receptor-regulated
  Smad (R-Smad), ortholog of vertebrate SMAD1/5/9 PMID:11703946.
• Cloned as a dominant enhancer of weak dpp alleles; "MAD is a member of a novel, highly conserved
  protein family" PMID:7768443.
• Domain architecture: N-terminal MH1 (DNA binding via a beta-hairpin), proline-rich linker,
  C-terminal MH2 (protein interaction, trimerisation) ending in the receptor-phosphorylated SSXS
  motif (Ser453/Ser455) [UniProt P42003 FT/CC].

Signal transduction mechanism

• Mad acts in responding cells, cell-autonomously, downstream of Dpp receptors
  [PMID:8681796 "Mad is an essential component of the signal transduction pathway downstream of the
  Dpp receptors in responding cells"; PMID:8681791 "MAD is required specifically in cells responding to
  DPP"].
• Phosphorylation by Tkv induces homo-oligomerisation and hetero-complex formation with Medea
  (Smad4) [PMID:9693372 "Mad forms a heteromeric complex with Drosophila Smad4 (Medea) upon
  phosphorylation by Thick veins (Tkv)"; "Mad exists as a monomer in the absence of Tkv stimulation"].
• Mad drives nuclear translocation of Medea [PMID:9502733 "Mad is required to actively translocate
  Medea into the nucleus"; PMID:9502724 "MEDEA ... requires physical association with MAD for nuclear
  translocation"].
• Besides Tkv/Sax, the activin receptor Baboon can phosphorylate Mad directly, held in check by
  dSmad2 competition [PMID:22563507, full text; PMID:18820452].
• Cytoplasmic in unstimulated cells PMID:8681791; with endogenous tagging only a minor fraction becomes nuclear
  [PMID:39494616, full text].

Transcription factor activity

• MH1 domain has sequence-specific DNA binding, revealed when the C-terminus is removed; Mad binds and
  is required for the vestigial quadrant enhancer PMID:9230443.
• Activation elements (AE) recruit Mad/Medea; Brinker competes for binding [PMID:20010841;
  PMID:11306550 full text "Brinker efficiently competes with Mad in vitro"].
• Silencer elements (SE): a trimeric Mad/Medea complex (two Mad, one Med) recruits the zinc-finger
  protein Schnurri to repress brinker, bam and other genes [PMID:12705870; PMID:17008448; PMID:16109720
  per UniProt]. This is the main mechanism of Dpp gradient read-out in the wing.
• Shn binds Mad directly and they act synergistically PMID:11071761.
• In motor neurons, both AE- and SE-type motifs mediate direct activation by pMad/Med, independent of
  Shn and Brk [PMID:36029115, full text].
• Negative feedback on dpp transcription via BMP-SE motifs [PMID:40955849, full text].

Regulation of Mad

• E3 ligase Smurf binds and degrades phosphorylated Mad [PMID:11703946; PMID:12754252].
• eIF4A promotes activation-dependent Mad/Medea degradation PMID:17115029.
• PDP phosphatase dephosphorylates the SSXS motif PMID:16510868.
• Linker phosphorylation by CDK8 and Shaggy/GSK3 limits signal intensity and range [PMID:25377173,
  full text]; Yorkie binds linker-phosphorylated Mad [PMID:19914168, full text].
• Nemo phosphorylates Ser25 and promotes nuclear export [PMID:17507407; PMID:19451277 at the NMJ].
• Lis1 binds and stabilises Mad in germline stem cells PMID:21041636.
• Nuclear accumulation requires nucleoporin Nup154 PMID:21696798.
• The Ski/Sno-family protein Fussel binds Medea, not Mad, and impairs Mad/Medea complex function
  without blocking pMad nuclear translocation [PMID:22879948, full text: "we could not verify an
  interaction of Fuss with Mad on an in vitro level"; "Fuss does not inhibit the nuclear translocation
  of pMad"].

Biological contexts (all downstream of BMP signalling)

Embryonic dorsal-ventral patterning, midgut/visceral mesoderm induction, wing and other imaginal disc
patterning and growth, eye morphogenetic furrow initiation, pupal retina patterning, R7/R8 subtype
pairing (with dSmad2) PMID:28853393, ovarian germline and somatic stem cell maintenance
[PMID:9695953; PMID:16256740; PMID:21041636], intestinal stem cell regulation [PMID:20056890;
PMID:23733344], retrograde BMP signalling for NMJ growth [PMID:15046722; PMID:22959350; PMID:19451277],
Ap4/FMRFa neuron specification PMID:26092715, histoblast spreading PMID:20226662.
These are contexts in which Mad acts as the BMP transcriptional effector; they are pleiotropic outcomes
rather than distinct molecular functions.