The function and stability of collagens depend on the accurate triple helix formation of three distinct polypeptide chains. Disruption of this triple-helical structure can result in connective-tissue disorders. Triple helix formation is thought to depend on three-stranded coiled-coil oligomerization sites within non-collagenous domains. However, only little is known about the physiological relevance of these coiled-coil structures. Transmembrane collagen XVII, also known as 180 kDa bullous pemphigoid antigen provides mechanical stability through the anchorage of epithelial cells to the basement membrane. Mutations in the collagen XVII gene, COL17A1, cause junctional epidermolysis bullosa (JEB), characterized by chronic trauma-induced skin blistering. Here we exploited a novel naturally occurring COL17A1 mutation, leading to an in-frame lysine duplication within the coiled-coil structure of the juxtamembranous NC16A domain of collagen XVII, which resulted in a mild phenotype of JEB due to reduced membrane-anchored collagen XVII molecules. This mutation causes structural changes in the mutant molecule and interferes with its maturation. The destabilized coiled-coil structure of the mutant collagen XVII unmasks a furin cleavage site that results in excessive and non-physiological ectodomain shedding during its maturation. Furthermore, it decreases its triple-helical stability due to defective coiled-coil oligomerization, which makes it highly susceptible to proteolytic degradation. As a consequence of altered maturation and decreased stability of collagen XVII trimers, reduced collagen XVII is incorporated into the cell membrane, resulting in compromised dermal-epidermal adhesion. Taken together, using this genetic model, we provide the first proof that alteration of the coiled-coil structure destabilizes oligomerization and impairs physiological shedding of collagen XVII in vivo.