IN-VIVO MICROTUBULE DYNAMICS DURING EXPERIMENTALLY-INDUCED CONVERSIONS BETWEEN TUBULIN ASSEMBLY STATES IN ALLOGROMIA-LATICOLLARIS

A distinctive property of foraminiferan tubulin is that, in addition t o microtubules (MTs), it exists in an alternate assembly state, helica l filaments. Here, we have examined in vivo MT dynamics during experim entally induced conversions between these two assembly states in the r eticulopods of the marine foraminiferan Allogromia laticollaris. Expos ure to high extracellular concentrations of Mg2+ (165 mM) resulted in a complete conversion of MTs into helical filaments. However, Mg2+ tre atment also induced a retrograde movement of organelles and cytoplasm, and it was necessary to inhibit this response in order to assess the effects of assembly state changes on individual MTs. This was accompli shed by simultaneous treatment with high extracellular Mg2+ and 2,4-di nitrophenol (DNP). The resulting loss in MTs was detected by video enh anced DIC (VEC-DIC) microscopy as either an endwise MT shortening (at an average rate of 474 mu m/min) or transformation into one or more ir regularly shaped fibrils, which we termed residual fibrils. Correlativ e immunofluorescence and video microscopy showed residual fibrils to b e composed of helical filaments. Removal of extracellular Mg2+/DNP ini tiated a reversal in assembly state, from helical filaments into MTs, which was completed within 5 min. VEC-DIC microscopy showed that MTs r eformed by an endwise lengthening at an average rate of 216 mu m/min. These results suggest that conversion between alternate tubulin assemb ly states provides a more rapid means to build and dismantle MTs than conventional subunit-driven pathways. (C) 1996 Wiley-Liss, Inc.