To identify the intramuscular signal transduction pathways that regulate proteolysis, we have used transgenic strains of C. elegans which express a soluble, enzymatically active
unc-54::lacZ fusion protein in 103 body-wall and vulval muscle cells. The reporter protein is completely stable in fed wild-type animals. Reporter degradation by a pre-existing proteolytic system is triggered in fed adults when the Raf-MEK-MAPK cascade is activated in response to acute signals from LET-60 Ras or EGL-15 FGF receptor, but not from LET-23 EGFR. FGFR signals via Ras-Raf-MEK-MAPK to promote proteolysis. In contrast to degradation induced by starvation or denervation, this is not prevented by proteasome inhibitors. Protein degradation was also provoked by reduced IGFR signaling following temperature upshift of
daf-2(
m41ts) or
age-1(
hx546ts) adults or by treatment of wild-type adults with the PtdIns-3-kinase (AGE-1) inhibitor LY-290042 (LY). All of these effects can be triggered in the presence of cycloheximide and almost certainly represent non-transcriptional outputs from the signaling pathways. Two lines of evidence show that the signals are intramuscular: a) muscle-specific expression of
daf-2+ prevented protein degradation in
daf-2(rf) mutants; b) muscle-specific expression of
age-1+ prevented degradation in an
age-1(
mg44) background, but did not do so in the presence of LY. LY-induced degradation was also prevented by mutations (
daf-18(rf),
pdk-1(gf),
akt-1(gf)) that increase signal downstream of AGE-1 or by mutations (
lin-45(rf),
mek-2(rf),
mpk-1(rf)) that block signaling downstream of Ras. Conversely, increased activity of the DAF-2 pathway (
daf-18(rf)) blocked protein degradation induced by EGL-15 activation. Single mutations (
let-756 or
egl-17) affecting FGF-like ligands did not suppress LY-induced degradation, but there was good suppression in a
let-756;
egl-17 double mutant, implying that LET-756 and EGL-17 signal redundantly to activate EGL-15 FGFR. Thus, our results suggest that DAF-2 signals via AGE-1, PDK-1 and AKT-1 to inhibit LIN-45 Raf and protein degradation in muscle. They also imply that in normal adult muscle there is degradation-promoting signal from FGFR via Ras to Raf, but this signal is not passed to subsequent steps in the cascade (and protein is not degraded) unless the balancing Akt inhibition of Raf is released in consequence of low IGFR signal. This offsetting balance between two signaling pathways with distinct inputs likely enables muscle to respond to a variety of physiological changes. NJS Present address: NASA Ames Research Center, Moffett Field, CA