Microtubules, intermediate filaments, and actin filaments

Microtubules are part of the cytoskeleton and can assemble into bundles, for example, the cortical microtubules, the prophase band, the spindle, and the phragmoplast. Cortical microtubules can appear in proximity to the plasmalemma and may be attached to it via cross bridges. These microtubules seem to be involved in orienting cellulosic microfibrils in the wall. Transmission electron micrographs revealed microtubules to be elongated hollow tube-like structures. Biochemistry demonstrated that the protofilaments are composed of tubulin dimers formed by the polymerization of α- and β-tubulins. Microtubules can polymerize and depolymerize at both the plus (β-tubulin) and minus (α-tubulin) ends, a process termed "dynamic instability". There are extended F-actin networks that are composed of F-actin bundles. In addition, G-actin can assemble as actin oligomers consisting of a few actin molecules. The functions of actin filaments are diverse. Of recent research interest is the perception-signaling roles of plant cells.