Elongation of the C. elegans embryo depends on microfilament-mediated cell shape changes within the hypodermis1. We had previously isolated two genes that affect this process. Strong
let-502 alleles lead to arrest at the onset of elongation, whereas mutations in
mel-11 can suppress this elongation defect.
let-502 encodes a Ser/Thr kinase (homolg of human Rho-kinase) predicted to be activated by the small Ras-related GTPase Rho.
mel-11 encodes the regulatory subunit of a smooth muscle myosin phosphatase. In analogy to proposed signal transduction events in smooth muscle contraction, we have suggested that Rho-activated LET-502 inactivates the MEL-11 myosin phosphatase complex. This in turn would lead to increased phosphorylation levels of myosin regulatory light chain by myosin light chain kinase, resulting in the cell shape changes necessary for embryonic elongation2. We have recently identified several genes that appear to interact with
let-502 and/or
mel-11 (see also abstracts by Mains & Wissmann; Piekny et al.). These include a guanine exchange factor (
unc-73), a Rac-like GTPase (
mig-2) and a dominant suppressor of
unc-73 (
sup-39). Guanine exchange factors (GEF's) promote the exchange of GDP for GTP among the small GTPases (such as Ras, Rho, Rac and Cdc42), thus increasing the relative amount of the activated form of these GTPases.
unc-73 contains two such guanine exchange factor domains. One of these appears to be specific for Rho-like GTPases, whereas the other is specific for Rac-like GTPases3. In our analysis we have used several alleles of
unc-73 (
e936,
gm40,
rh40; many thanks to Rob Steven and Joe Culotti). All three alleles enhance the embryonic lethality associated with
mel-11 mutations. Of the three, the partial loss-of-function mutation
e936 shows the weakest effect.
gm40 and
rh40 both show drastic effects in our assay.
gm40 most likely is a null, whereas
rh40 is a missense mutation in the Rac-specific GEF domain. The latter is particularly interesting since it suggests that Rac serves to activate
mel-11. Indeed, mutations in the Rac-like GTPase
mig-2 (kindly provided by Rachel Kindt and Cynthia Kenyon), which had been shown to interact with
unc-734, enhance
mel-11. Recently, we have found that the dominant
unc-73 suppressor
sup-39 also suppresses
mel-11 and enhances
let-502 mutations. These data suggest that we have identified at least two signal transduction pathways active during embryonic elongation. Rho activates LET-502 thereby promoting embryonic elongation. Rac activates MEL-11, which by antagonizing LET-502 inhibits embryonic elongation. Thus it appears that the two GTPases Rho and Rac have opposing roles during embryonic elongation. This is in contrast to previous work by other groups in which Rho and Rac were thought to act together in a signal cascade. 1 Priess & Hirsh (1986) Dev. Biol. 117, 156-173 2 Wissmann et al. (1997) Genes & Dev. 11, 409-422 3 Steven et al. (1998) Cell 92, 785-795 4 Zipkin et al. (1997) Cell 90, 883-894