Laminin 5 Binds the NC-1 Domain of Type VII Collagen

Mutational analyses of genes that encode components of the anchoring complex underlying the basolateral surface of external epithelia indicate that this structure is the major element providing for resistance to external friction. Ultrastructurally, laminin 5 (α3β3γ2; a component of the anchoring filament) appears as a thin filament bridging the hemidesmosome with the anchoring fibrils. Laminin 5 binds the cell surface through hemidesmosomal integrin α6β4. However, the interaction of laminin 5 with the anchoring fibril (type VII collagen) has not been elucidated. In this study we demonstrate that monomeric laminin 5 binds the NH₂-terminal NC-1 domain of type VII collagen. The binding is dependent upon the native conformation of both laminin 5 and type VII collagen NC-1. Laminin 6 (α3β1γ1) has no detectable affinity for type VII collagen NC-1, indicating that the binding is mediated by the β3 and/or γ2 chains of laminin 5. Approximately half of the laminin 5 solubilized from human amnion or skin is covalently complexed with laminins 6 or 7 (α3β2γ1). The adduction occurs between the NH₂ terminus of laminin 5 and the branch point of the short arms of laminins 6 or 7. The results are consistent with the presumed orientation of laminin 5, having the COOH-terminal G domain apposed to the hemidesmosomal integrin, and the NH₂-terminal domains within the lamina densa. The results also support a model predicting that monomeric laminin 5 constitutes the anchoring filaments and bridges integrin α6β4 with type VII collagen, and the laminin 5–6/7 complexes are present within the interhemidesmosomal spaces bound at least by integrin α3β1 where they may mediate basement membrane assembly or stability, but contribute less significantly to epithelial friction resistance.