Sea-urchin tubulins can be assembled in vitro to form microtubules provided that a suitable seed is present. Although fragments of sea-urchin microtubules fail to act as seeds, since they apparently dissociate, short pieces of rat brain microtubule will act as nucleation sites for the assembly of sea-urchin tubulins. In addition, rat brain tubulins appear to undergo a certain amount of in vitro assembly when seeded with fragments of microtubules derived from sea-urchin sperm tails. Assembly on a heterologous seed indicates that the current in vitro assembly conditions may be insufficiently stringent.

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