The nematocyst wall of cnidarians is a unique biomaterial that withstands extreme osmotic pressures, allowing an ultrafast discharge of the nematocyst capsules. Assembly of the highly robust nematocyst wall is achieved by covalent linkage of cysteine-rich domains (CRDs) from two main protein components, minicollagens and nematocyst outer wall antigen (NOWA). The bipolar minicollagens have different disulfide patterns and topologies in their N and C-terminal CRDs. The functional significance of this polarity has been elusive. Here, we show by NMR structural analysis that all representative cysteine-rich domains of NOWA are structurally related to N-terminal minicollagen domains. Natural sequence insertions in NOWA CRDs have very little effect on the tightly knit domain structures, nor do they preclude the efficient folding to a single native conformation. The different folds in NOWA CRDs and the atypical C-terminal minicollagen domain on the other hand can be directly related to different conformational preferences in the reduced states. Ultrastructural analysis in conjunction with aggregation studies argues for an association between the similar NOWA and N-terminal minicollagen domains in early stages of the nematocyst wall assembly, which is followed by the controlled association between the unusual structures of C-terminal minicollagen domains.
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