Proper dendrite morphogenesis is important for the establishment of functional neuronal circuitry, as evidenced by the defects in dendrite shape seen in human neurological disease. In a screen for genes required for shaping the elaborate dendritic PVD arbors in C. elegans, we have identified alleles of
kpc-1, which encodes a proprotein convertase homolog of the
kex2/furin family. Previous findings indicated that: 1.KPC-1 promotes branch extension and suppresses 2deg branch formation, and requires convertase activity for this process. 2.KPC-1 expressed specifically in PVD is sufficient to rescue the mutant phenotype. 3.KPC-1 exhibits widespread neuronal expression, including in PVD neurons. Genetic interactions further indicated that
kpc-1 acts in concert with the previously described menorin pathway, but whether this interaction is attributable to direct or indirect processing of MNR-1 or DMA-1 by KPC-1 remained unknown. To further dissect the pathway by which
kpc-1 influences dendritic architecture, we pursued an unbiased proteomic approach to identify potential KPC-1 substrates and/or downstream interactors. Reasoning that the absence of a protease may manifest in a changed peptide profile, we compared the proteomes of wild-type animals,
kpc-1 mutants and cell-specifically-rescued mutants. We identified 130 proteins that were differentially expressed in the mutant, but indistinguishable between the transgenically rescued strain and wild type animals. RNAi and mutant analysis of these candidates identified
mig-6/papilin, which encodes a large ECM protein.
mig-6 loss of function mutations and RNAi experiments demonstrated that, similar to
kpc-1, it functioned in proper 1deg branch localization suggesting a potential role as a substrate/effector of KPC-1 activity. Genetic characterization of
mig-6 established that the observed phenotype is the result of haploinsufficiency, whereas missense mutations in conserved cystein residues in the lagrin repeat domains three and four produced a dominant antimorphic effect. Trans-heterozygous animals for
mig-6 and
kpc-1 showed enhanced defects in 1deg branch localization compared to heterozygous singles mutants indicating that both genes genetically interact. Overall these data are consistent with a stepwise model for dendrite development where different sets of genes affect different steps of dendritc arbor formation and where
kpc-1 seems to be involved at different levels during this process.