Mutations in the collagen XVII gene, COL17A1, are associated with junctional epidermolysis bullosa. Most COL17A1 mutations lead to a premature termination codon (PTC), whereas only a few mutations result in amino acid substitutions or deletions. We describe here two novel glycine substitutions, G609D and G612R, and a splice site mutation resulting in a deletion of three Gly-X-Y amino acid triplets. In order to investigate the molecular pathomechanisms of non-PTC mutations, G609D and G612R and two previously known substitutions, G627V and G633, and deletion of the amino acids 779-787 were introduced into recombinant collagen XVII. The thermal stability of the mutated collagens was assessed using trypsin digestions at incremental temperatures. All the four glycine substitutions significantly destabilized the ectodomain of collagen XVII, which manifested as 16 degrees C-20 degrees C lower T(m) (midpoint of the helix-to-coil transition). These results were supported by secondary structure predictions, which suggested interruptions of the collagenous triple helix within the largest collagenous domain, Col15. In contrast, deletion of the three full Gly-X-Y triplets, amino acids 779-787, had no overall effect on the stability of the ectodomain, as the deletion was in register with the triplet structure and also generated compensatory changes in the NC15 domain.