A cytoskeletal 50 kDa protein in higher plants that forms intermediate-sized filaments and stabilizes microtubules

Summary Filamentous structures of 7–10 nm in diameter were regenerated in vitro from a soluble 50 kDa protein (p50) that had been isolated from mung bean seedlings and from cultured tobacco cells. The polymerization of p50 in vitro was dependent on the presence of guanosine nucleotides, in particular, guanosine monophosphate (GMP). Unlike tubulin, p50 is a stable basic protein with the ability to polymerize even after heat treatment for 1 min at 70 °C. Furthermore, the freeze-dried powder of p50 retained the ability to regenerate filamentous structures after it had been dissolved in polymerization buffer to which GMP was then added. Two monoclonal antibodies against p50 were obtained. These antibodies stained the filamentous structures that extended from the surface of the nucleus to the cell periphery in interphase tobacco cells. They stained spindles and phragmoplasts as did tubulin-specific antibodies. They also stained fibrillar structures that were present around the spindle poles and the telophase daughter nuclei in which no microtubules were present. These results suggest that a part of the cell's complement of p50 may be associated with microtubules in dividing cells while the rest may itself form unique fibrillar structures. The antibodies against p50 did not stain cortical microtubules or the pre-prophase band of microtubules. The antibody against p50 also stained intermediate filament-like structures in cultured animal cells. The formation of microtubules in vitro was markedly stimulated and the assembled microtubules were greatly stabilized by p50. Further investigation of p50 is indispensable for the understanding of properties and function of intermediate-sized filaments in higher plant cells.Abbreviations EPC Sepharose ethyl N-phenyl-carbamate conjugated Sepharose - p50 50 kDa protein