Microtubule gelation-contraction in vitro and its relationship to component a of slow axonal transport

Microtubule proteins, isolated by cycles of assembly, will undergo ATP-dependent gelation-contraction in vitro. A particulate component is present in these preparations, which is required for the gelation-contraction of microtubules assembled from purified tubulin. These particulates contain tubulin, neurofilament, spectrin, MAP2, and other as yet unidentified proteins. The particulates have a microtubule-stimulated ATPase that may be unique and is the likely motor for microtubule gelation-contraction. The basic structural unit of these particulates appears to be a crescent-shaped, or hemispherical, granule about 20 nm in diameter. The particles move along microtubule walls at a rate of about 1 micron. When compared to known physiological phenomena, microtubule gelation-contraction has striking similarities to component a of slow axonal transport (SCa), but displays no relationship to slow component b or to fast transport. On the basis of their similarities in composition, solubility, and rate of movement, we have proposed that the particulates responsible for microtubule gelation-contraction are the insoluble protein complexes, which have been suggested to be the transported component of SCa. We have termed these structures "slow component a particulates" or "SCAPs." It is probable that similar motile protein complexes exist in cells other than neurons, and we propose the term "dynasome" to describe such structures in general.