The distal tip cells (DTCs) migrate in a U-shaped pattern along the body wall basement membrane during development of the hermaphrodite gonad. An ADAM family metalloprotease MIG-17 is secreted from the body wall muscles and localizes to the gonadal basement membrane where it is required for directional migration of DTCs. To identify the molecules interacting with MIG-17, we have isolated suppressors of a putative null allele
mig-17(
k174). fibulin(fbl)-1 mutations,
fbl-1(
k201) and
fbl-1(
k206), are semi-dominant, gain-of-function (gf) suppressors of
mig-17. Most
fbl-1(gf);
mig-17(
k174) double mutant animals exhibit U-shaped gonad morphology as do the wild-type animals.
fbl-1 encodes two proteins highly homologous to fibulin-1 isoforms FBL-1C and FBL-1D, Ca2+-binding extracellular matrix proteins. Specific amino acid substitutions in the third EGF-like motif of one of the two isoforms, FBL-1C, which corresponds to mammalian fibulin-1C, suppress
mig-17 mutations. FBL-1C is synthesized in the gut cells and localizes strongly to the gonadal basement membrane in a MIG-17-dependent manner (Kubota et al., 2004). A loss-of-function mutation,
fbl-1(
q750), suppresses the gonadal defects of a null mutant of
gon-1(
q518) (another secreted ADAM protein), but not
mig-17 (Hesselson et al., 2004). We found that the
fbl-1(
k201) gf mutation also suppresses the weak
gon-1(
e1254) allele.
fbl-1(
k201) significantly suppressed the gonadal defects of
gon-1(
e1254): almost half of the animals formed normal U-shaped gonad arms and were fertile. Since mammalian FBL-1C binds strongly to nidogen-1, a basement membrane protein, in vitro (Sasaki et al. 1995), we examined the interaction between FBL-1 and NID-1 to elucidate the mechanisms of suppression of
mig-17 and
gon-1 by gf-type FBL-1 proteins. Interestingly, the introduction the null
nid-1(
cg119) allele into
fbl-1(
k201);
mig-17(
k174) abolished the suppression effect of
fbl-1(
k201). Similar result was observed in
fbl-1(
k201)
gon-1(
e1254);
nid-1(
cg119) triple mutants. These results indicate that the FBL-1
(k201) mutant protein suppresses both
mig-17 and
gon-1 in a wild-type NID-1 dependent manner. Therefore, it is likely that the FBL-1
(k201) protein can mimic the downstream event common to the actions of MIG-17 and GON-1 which is required for correct DTC migration. In a parallel study, we are trying to identify the molecules interacting with wild-type and mutant FBL-1C proteins using immunoprecipitation analysis.