The cytoskeleton of Reticulomyxa filosa reticulopodia contains Glu-tubulin as a main component

Abstract

Reticulomyxa filosa and the foraminifer Allogromia laticollaris share not only highly dynamic reticulopodia but also a unique cytoskeleton. Similar to Allogromia, the pseudopods of R. filosa contain numerous colinear microtubule (MT)-bundles, which under normal conditions or after experimental stimulation, became transformed into different polymorphic tubulin assemblies, such as helical filaments (hF), linear filaments (Fl) or even paracrystals (PCs). The actin filaments of R. filosa reticulopodia are parallel and enmesh the MT-cables over long distances. As shown after decoration with HMM, their orientation along MT-trunks is unidirectional and at the lamellipodial front, we detected a plaque-like arrangement. Indirect immunofluorescence (IIF) with monoclonal antibodies directed against tyrosinated α-tubulin (Tyr-tubulin) and against detyrosinated α-tubulin (Glu-tubulin) shows clearly that the reticulopodial MTs of R. filosa are exclusively composed of Glu-tubulin. Staining with YOL-34 antibody, directed against fixed tubulin, also reveals the existence of a second MT-subpopulation. Long term treatment (40 h) with 10 μM taxol leads to a remarkable reduction of the Glu-tubulin, favoring the formation of Tyr-MTs. This can be taken to indicate synthesis of new tubulin. Since electrophoresis and western blotting showed no uniformity between the Allogromia and R. filosa tubulin, their posttranslationally modified α-tubulins may be - beside some unknown factors - an important prerequisite for the observed transformational changes. The majority of cytoskeletal MTs never undergo complete disassembly, but reform from a stable intermediary disassembly state (e.g., helical filaments), since Glu-tubulin is thought to be typical for "old" tubulin polymers.