Worm Breeder's Gazette 14(2): 64 (February 1, 1996)

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.

Genetic interactions between sli-1 and let-60 ras.

Charles H. Yoon, Paul W. Sternberg.

HHMI and Division of Biology, 156-29, Caltech, Pasadena, CA 91125

sli-1 is a negative regulator of let-23-mediated vulval induction and
encodes a protein similar to the mammalian proto-oncoprotein c-Cbl (1). 
Reduction-of-function (rf) mutations in sli-1 suppress the vulvaless (Vul)
phenotype associated with severe reduction-of-function, but not null,
mutations of let-23 (2).  let-60 ras acts downstream in the signal
transduction process initiated by LET-23.  Initial studies have shown that
sli-1 has only a weak suppression of the slight Vul phenotype associated
with the let-60(rf) mutation, n2021 (2).  We have furthered this study to
more exactly determine how sli-1 interacts with let-60.
We used four severe let-60(rf) alleles defined by Beitel et al.(3): n1876,
n2022, n2034, n2035.  Animals bearing any of these four let-60(rf) alleles
display partial lethality as segregants from heterozygous mothers. 
Lethality is essentially 100 % for F2 homozygotes of these four alleles,
indicating that the partial penetrance of lethality in F1 homozygotes is
due to maternal rescue from the let-60(+) wild-type copy in the
heterozygous mothers.  We scored vulval induction in F1 homozygotes of
these four let-60(rf) alleles in sli-1(+) and sli-1(rf) backgrounds.  We
used the sli-1(rf) reference allele, sy143.  We find that sli-1(sy143)
does not suppress the Vul phenotype associated with severe let-60(rf)
mutations.  For example, n2035 is suppressed from 0.016 induced
VPCs/animal (n=62) in a sli-1(+) background to 0.034 induced VPCs/animal
(n=87) in a sli-1(sy143) background.
We tested whether sli-1 had any effect on the penetrance of lethality in
F1 homozygotes of the four let-60(rf) alleles coming from
unc-24(e138)let-60(rf)/dpy-20(e1282) heterozygous mothers in both sli-1(+)
and sli-1(sy143) backgrounds.  We find that sli-1(sy143) strongly
suppresses the lethality associated with the F1 homozygotes of the four
let-60(rf) alleles, n1876, n2022, n2034, and n2035.  For example,
sli-1(sy143) increased the survival of F1 n2035 homozygotes to 86 % from
the 20 % F1 survival in a sli-1(+) background.  However, lethality is
still 100 % for F2 homozygotes in a sli-1(sy143) background.  This
indicates that sli-1's suppression of F1 homozygote lethality is an
enhancement of the maternal rescue by the wild-type copy of let-60 in the
heterozygous mothers and that sli-1 does not suppress the lethality
associated with severe let-60(rf) alleles.  This is in contrast to sli-1's
strong suppression of lethality associated with severe let-23(rf)
mutations (2).
Since sli-1 does not suppress null alleles of let-23, we generated a
series of hetero-allelic let-60(rf) animals and scored for vulval
induction in F1 to test whether the four let-60(rf) alleles tested above
were in fact null alleles.  We find that n1876, n2022, and n2035 are
non-null for vulval induction.  For example, n2035/n2021 hetero-allelic F1
animals have 1.6 induced VPCs/animal whereas sy127/n2021 animals have 0.8
induced VPCs/animal and sDf8/n2021 animals have 0.6 induced VPCs/animal. 
sy127 is a putative null allele of let-60 and sDf8 is a deficiency which
uncovers let-60.
In summary, our results indicate that sli-1(rf) mutations do not suppress
the Vul phenotype of severe, non-null let-60(rf) mutations and therefore
suggests that, in vulval induction, SLI-1's target of action lies upstream
of LET-60.

1.  Yoon, C. H., Lee, J., Jongeward, G. D. and Sternberg, P. W.  (1995) 
Science  269: 1102-1105.
2.  Jongeward, G. D., Clandinin, T. R. and Sternberg, P. W.  (1995) 
Genetics  139: 1553-1566.
3.  Beitel, G. J., Clark, S. G. and Horvitz, H. R.  (1990)  Nature  348: