The capsule, actually a specialized organelle called a nematocyst, is used by Hydra for defense, locomotion, and prey capture. Nematocysts contain a high concentration of poly-γ-glutamate, resulting in an internal pressure of up to 150 atmospheres that drives the explosive discharge of an internal spiny tubule when the organism is irritated or detects prey. This discharge has a maximum velocity of 2 m per second and an acceleration of 40,000 g, and is thus one of the fastest cellular processes in nature.
Nematocysts must have a strong wall to contain such pressure. Jürgen Engel (University of Basel, Basel, Switzerland) and colleagues suspected that disulfides might be important, as reducing conditions cause nematocysts to burst. They examined disulfide formation in minicollagen-1, a nematocyst wall component, and found that minicollagen-1 synthesized either recently in vivo, or at any time in a recombinant system, has only intramolecular disulfides. A switch from intramolecular to intermolecular disulfides, detected biochemically and as a reduction in antibody reactivity, was associated with hardening of the nematocyst wall. This process may be similar to the acquisition of intermolecular disulfides by mammalian collagen IV during basement-membrane formation. ▪