Naturally occurring tubulin-containing paracrystals in Allogromia: immunocytochemical identification and functional significance.

Bundles of microtubules (MTs) are readily visualized in vivo by videomicroscopy in highly flattened reticulopodia of the foraminiferan protozoan Allogromia sp. strain NF. In this report we use videomicroscopy, immunocytochemistry, and high-voltage electron microscopy to characterize the dynamic changes that occur in this extensive MT cytoskeleton, and in the associated cytoplasmic transport, during induced withdrawal and subsequent reextension of reticulopodia. Within seconds after application of the withdrawal stimulus (seawater substitute made hypertonic with MgCl2) intracellular bidirectional transport along linear MT-containing fibrils ceases and is replaced by an inward, constant-velocity flow of cytoplasm along the fibrils. As withdrawal continues, most fibrils become wavy and coalesce to form phase-dense pools. These wavy fibrils and phase-dense pools contain a paracrystalline material and few if any MTs. Same-section correlative immunofluorescence and high-voltage electron microscopy reveal that the paracrystalline material contains tubulin. During recovery linear fibrils (MTs) rapidly extend from the phase-dense pools (paracrystals), which concurrently shrink in size, thus reestablishing normal network morphology and motility. We conclude that the MT cytoskeleton in Allogromia reticulopodia is transformed during withdrawal into a tubulin-containing paracrystal, which serves as a temporary reservoir of MT protein and an initiation site for MT regrowth.