C. elegans has a pair of U-shaped gonad arms. The shape of the gonad arms reflects the migration paths of the gonadal distal tip cells (DTCs) during larval development. The migration of DTCs is divided into 3 phases: (1) migration along the A-P axis on the ventral muscle, (2) migration along the D-V axis across the lateral hypodermis and (3) migration along the A-P axis on the dorsal muscle. The DTCs in
mig-22 and
mig-26 mutants have defects in the second phase or dorsal migration, but the first and third phases are essentially normal. They often do not or only partially execute the second phase. Thus, both
mig-22 and
mig-26 mutants appear to be defective in the guidance of DTCs along the D-V axis. To examine whether
mig-22 and
mig-26 interact with known D-V guidance molecules, double mutants were made with a weak allele
unc-6(
e78) or a null allele
unc-129(
ev554) that have very weak or no effect on the second phase DTC migration. These double mutants strongly enhanced the dorsal migration defects of the posterior DTCs, while partially suppressed those of the anterior, suggesting the functions of
mig-22 and
mig-26 in these guidance mechanisms. Molecular analysis revealed that MIG-26 is a type II transmembrane protein highly homologous to human chondroitin synthase, a glycosyltransferase. Although a novel protein, MIG-22 has weak homology with MIG-26. The amounts of chondroitin in both
mig-22 and
mig-26 mutants were found to be significantly reduced, implying that both
mig-22 and
mig-26 are required for the chondroitin biosynthesis. To identify the cells expressing
mig-22, a functional HA-tag fusion gene
mig-22::HA was introduced into
mig-22 mutant. The expression of
mig-22::HA was detected in DTCs from the L3 thorough the young adult stages and the uterus in the L4 stage. Furthermore, the
mig-22 expressed under the DTC specific
lag-2 promoter rescued the defects of DTC migration of
mig-22, indicating that MIG-22 is required cell autonomously for migration of DTCs. To investigate cellular localization, we are raising antibodies against MIG-22 and MIG-26. These results suggest that chondroitin proteoglycans plays critical roles in the UNC-6/netrin and UNC-129/TGF- guidance systems.