The gonad arms of C. elegans hermaphrodites acquire invariant shapes by guided 
migrations of distal tip cells (DTCs), which occur in three phases that differ 
in the direction and basement membrane substrata used for movement. We found 
that mig-6 encodes long (MIG-6L) and short (MIG-6S) isoforms of the 
extracellular matrix protein papilin, each required for distinct aspects of DTC 
migration. Both MIG-6 isoforms have a predicted N-terminal papilin cassette, 
lagrin repeats and C-terminal Kunitz-type serine proteinase inhibitory domains. 
We show that mutations affecting MIG-6L specifically and cell-autonomously 
decrease the rate of post-embryonic DTC migration, mimicking a post-embryonic 
collagen IV deficit. We also show that MIG-6S has two separable functions - one 
in embryogenesis and one in the second phase of DTC migration. Genetic data 
suggest that MIG-6S functions in the same pathway as the MIG-17/ADAMTS 
metalloproteinase for guiding phase 2 DTC migrations, and MIG-17 is abnormally 
localized in mig-6 class-s mutants. Genetic data also suggest that MIG-6S and 
non-fibrillar network collagen IV play antagonistic roles to ensure normal phase 
2 DTC guidance.