Direct Transformation of Microtubules into Helical Filaments by Low Salt Concentrations in Reticulomyxa filosa

Ralf N. Breuker and Manfred Hauser, Lehrstuhl für Zellmorphologie, Ruhr-Universität Bochum, 44780 Bochum, FRG

Microtubules are the main component of the reticulopodial networks (RPNs) of most foraminifers and the closely related fresh water organism Reticulomyxa. They share remarkable high dynamics in the assembly and disassembly of their microtubule-cytoskeletons. Obviously, they have developed specific mechanisms for this fast rearrangement, which are at least ten times faster as in most other organisms [1]. This may be partly due to the fact, that their microtubular systems reveal not a complete disassembly into heterodimeric subunits, but only a partial disassembly into helical filaments respectively paracrystals [2].
We succeeded in transforming the microtubules of the RPN of Reticulomyxa directly into helical filaments by extracellular application of 20 mM MgCl2. Further investigation showed, that the presence of magnesium in the transformation assay is not essential, since a variety of cations with approximately the same ionic strength led to similar results. The transformation is reversible by rinsing with salt-free media, since cytoplasmic streaming and also organelle transport was immediately reconstituted.
We conclude, that the microtubules of Reticulomyxa can be directly transformed into helical filaments or vice versa, as originally proposed for the foraminiferan Allogromia by Hauser and Schwab [3], and as recently confirmed by Welnhofer and Travis [4]. Since the reaction is independent of specific ions, we suggest a common pathway for the transformation to helical filaments by the action of cations on microtubules via the cell membrane.
References: 1. Chen, Y. T. & Schliwa, M. Cell Motil. Cytoskel. 17, 214-226 (1990). 2. Hauser, M., Lindenblatt, J. & Hülsmann, N. Eur. J. Protistol. 25, 145-157 (1989). 3. Hauser, M. & Schwab, D. Cytobiologie 9, 263-279 (1974). 4. Welnhofer, E. A. & Travis, J. L. Cell Motil. Cytoskel. 34, 81-94 (1996).

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