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 suppressor mutations of a putative null allele
mig-17 (
k174) . More than 30 suppressor mutations were isolated using EMS mutagenesis.
saf-1 ( S uppressor of A dam F amily defect) mutations are semi-dominant suppressors and can weakly suppress the
mig-17 mutants as a heterozygote. Most
saf-1;
mig-17 double mutants exhibit U-shaped gonad morphology as seen in the wild-type animals. Using a deficiency eDf19 deleting the
saf-1 locus on LG IV , we found that eDf19/+ did not suppress DTC migration defect of
mig-17 (
k174) , indicating that the
saf-1 mutants are gain-of-function (gf)-type mutations. We found missense mutations in the predicted gene F56H11.1 in
saf-1 (gf) mutants.
saf-1 encodes two proteins highly homologous to fibulin-1 isoforms, Ca 2+ -binding extracellular matrix proteins. To assess the function of gf-type and wild-type
saf-1 genes in DTC migration, we performed gene dosage analyses. When we introduced an extrachromosomal array with multi-copy gf-type
saf-1 genes into
mig-17 mutants, it suppressed migration defects as in the
saf-1 heterozygote. Interestingly, when we introduced a wild-type
saf-1 array into
saf-1;
mig-17 animals, it completely cancelled the suppression effect of the genomic gf-type
saf-1 alleles. These observations suggest that the wild-type SAF-1 protein interferes with directed DTC migration, while the gf-type SAF-1 allows it in the absence of MIG-17 protein. The
saf-1::Venus fusion gene revealed that SAF-1 accumulates to the basement membrane of the developing gonad where MIG-17 also localizes. A deletion mutant
saf-1 (
tk45) exhibited severe gonadal DTC migration defect and was sterile. These results indicate that the SAF-1 proteins interact with MIG-17 and play essential roles in guiding migration of the gonadal DTCs.