Worm Breeder's Gazette 17(1): 56 (October 1, 2001)
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
Dept. of Biological Sciences, Univ. of Pittsburgh, Pittsburgh PA 15260
· We reported earlier that activation of LET-60 Ras provokes the degradation of a soluble, enzymatically active unc-54::lacZ fusion protein in body-wall and vulval muscles [1]. Specifically, animals homozygous for the temperature-activated Ras allele ga89 shifted to 25°C as adults had a time-dependent loss of lacZ activity and of reporter protein. We also reported [2] that lin-45(sy96), mek-2(ku114), or mpk-1(n2521) suppressed degradation in let-60(ga89) animals. Using animals homozygous for gaIs37 (Ef1a::Dmek hs::mpk-1+) [3] we were able to acutely activate MPK-1 in adults and observed degradation of reporter protein. Thus, the Raf-MAPK cascade is necessary and sufficient to elicit muscle protein degradation in response to Ras.
We also reported that clr-1(e1745) animals, like let-60(ga89) animals, catabolized lacZ reporter at 25°C but not at 16°C. Degradation in clr-1(e1745) animals was suppressed by a reduction-of-function mutation in egl-15(n1783), which encodes a fibroblast growth factor receptor homologue. In clr-1(e1745), like let-60(ga89) animals, cycloheximide treatment from the time of temperature shift does not prevent protein breakdown, implying that in both cases protein catabolism does not depend upon induced gene expression, but rather uses pre-existing signaling pathways and proteases.
Since activation of EGL-15 (via clr-1 reduction-of-function), LET-60, or MPK-1 (gaIs37 animals) leads to both a "clear" phenotype and protein degradation, and egl-15(n1783) fails to suppress protein degradation in let-60(ga89) animals, we have performed epistasis experiments to determine if EGL-15 is signaling protein degradation via the Ras-MAPK pathway. A strong reduction-of-function mutation in let-60(n2021) was previously reported [4] not to suppress the "clear" phenotype of clr-1(e1745) and similarly fails to suppress protein degradation. However, treatment of these animals (but not clr-1(e1745)) with the Ras farnesyltransferase inhibitor manumycin [5] from the time of temperature shift does result in suppression of protein degradation and to a lesser extent the "clear" phenotype. Protein degradation in clr-1(e1745) animals is suppressed by lin-45(sy96), mek-2(ku114), or mpk-1(n2521). Both lin-45(sy96) and mek-2(ku114) also suppress the "clear" phenotype of these animals. Mutations in genes whose products have a role in Ras signaling and which are known to suppress the "clear" phenotype [sem-5(n1619), sem-5(n1779), soc-2(n1774), or sur-8(ku167)] also suppress protein degradation in clr-1(e1745) animals. The finding that both LET-60 and MPK-1 are necessary for protein degradation in clr-1(e1745) animals, coupled with the observation that activation of either LET-60 or MPK-1 is sufficient to provoke protein degradation, implies that EGL-15 signals protein degradation via SEM-5 and the Ras-MAPK pathway.
In contrast to our previous results on muscle protein degradation in response to starvation, loss of cholinergic input, or Ras activation, there is no precedent in mammals for FGFR activation leading to protein degradation. However, it is attractive to speculate that FGF-induced protein degradation may be important for muscle remodeling or myoblast migration. Currently we are attempting to determine how EGL-15 induced protein degradation is opposed by signal from another receptor tyrosine kinase known to be involved in protein catabolism in mammals, and if these signals are acting in muscle cells.
(Many thanks to Stuart Kim, Dave Eisenmann, Min Han and the CGC for strains.)
[1] Szewczyk and Jacobson, WBG 16(3): 29 (2000)
[2] Szewczyk & Jacobson, 2001 International Worm Meeting abstract 554
[3] Lackner and Kim, Genetics 150: 103-117 (1998)
[4] Schutzmann, Borland, Newman, Kokel & Stern, Mol. Cell. Biol. (in press)
[5] Hara and Han, PNAS 92: 3333-3337 (1995)