The microfilament-driven elongation of the C. elegans embryo1 requires the genes
let-502 and
mel-112.
let-502 and
mel-11 encode proteins similar to Rho-binding kinase and the regulatory subunit of myosin phosphatase, respectively. Genetic interactions suggested the following pathway:
let-502 (Rho kinase) _
mel-11 (Myosin phosphatase) _ Embryonic elongation This is consistent with the biochemically-defined roles of the vertebrate homologs of
let-502 and
mel-11 in smooth muscle, where myosin phosphatase blocks muscle contraction by dephosphorylating myosin light chain. In turn, Rho kinase activity can trigger contraction by inactivating myosin phosphatase. We are currently identifying more players in this pathway by isolating suppressors and by testing candidate genes (see abstracts by Wissmann et al. and Piekny et al.). In the course of building self-sterile strains of
let-502 and
mel-11, we blundered into the fact that
fem-2 is part of this pathway.
fem-2 behaves as a strong enhancer of
let-502. Whereas 1/4 of the progeny of
let-502(
ca201)/+ hermaphrodites arrest at elongation, the addition of
fem-2(
b245 or
e2105), results in the segregation of 3/4 defective offspring. Strong
let-502 hypomorphs are essentially wild-type by themselves, but with
fem-2, all larvae arrest with elongation defects. As expected from the above pathway, enhancers of
let-502 should suppress
mel-11. Indeed, while 2% of
mel-11(
it26) embryos hatch at 20_, 20% of those from
mel-11;
fem-2 are viable. How does
fem-2 act during elongation?
fem-2 and
mel-11 encode subunits of different classes of phosphatase complexes (PP2C3,4 and PP12, respectively). However,
fem-2 and
mel-11 genetically antagonize one another and so their corresponding phosphatase complexes cannot be acting redundantly. Instead,
fem-2(+) may function as an inhibitor of
mel-11(+), an activator or
let-502(+) or an activator of contraction by an independent pathway.
fem-2 likely acts independently of its role in sex determination. We found little or no genetic interactions when either
let-502 or
mel-11 are combined with
fem-1 or
fem-3 (we are currently examining interactions with the tra's). We have not noticed changes in sexual phenotype with
let-502 or
mel-11.
fem-2's activity during elongation must be redundant since we have observed only mild elongation defects in the absence of
let-502 mutations. We plan to isolate suppressors and enhancers of elongation in a
fem-2(ts) background to find more interacting genes. 1Priess and Hirsh (1986) 117:156 Devel. Biol. 2Wissmann et al. (1997) Genes Devel. 11:409 3Pilgrim et al. (1995) Molec. Biol. Cell. 6:1159 4Chin-Sang and Spence (1996) Genes Develop. 10:2314