Assembly and disassembly of plant microtubules: tubulin modifications and binding to MAPs

Abstract

Microtubules are dynamic heteropolymers of aand b-tubulin that assemble co-ordinately in response to a variety of intracellular and extracellular signals and participate in a number of different functions in eukaryotic cells, from cell division to organelle transport, from RNA positioning to flagellar beating. In plant cells, microtubules assemble and disassemble during the cell cycle to organize different microtubule arrays. Interphase cortical microtubules have a critical role in the construction of the cell wall by controlling the correct deposition of cell wall polymers (Lloyd and Chan, 2008). During cell division, microtubules are arranged into characteristic structures. The preprophase band (a circular array of microtubules) defines the future construction site of the cell plate, thus imposing asymmetry on the daughter cells. The mitotic spindle shares the function of the analogous structure of animal and fungal cells but its organization shows some critical differences, mainly caused by the absence of centrioles. The phragmoplast is a special microtubule array that substitutes the contractile ring of animal cells during cytokinesis, allowing the synthesis of a new cell wall that physically separates the two daughter cells. Since the four different microtubule arrays have distinct features and structures, use of different proteins (tubulin and non-tubulin) is a critical requisite for the assembly of each array. Understanding how individual proteins are used in the assembly of microtubules will allow a clearer picture of how microtubules perform their function and pass from interphase to mitotic arrays (and vice versa). In this issue, Jovanovic and colleagues (Jovanovic et al., 2010) report that the tyrosination/detyrosination cycle of tubulin could regulate the transition of plant cells from the elongation to the division stages. In their work, a specific compound (nitrotyrosine) is used irreversibly to incorporate tyrosine into detyrosinated a-tubulin; the consequence of this post-translational modification is the inhibition of mitosis and the increase in cell elongation. The article points to the importance of post-translational modification of tubulin in the reorganization of the microtubule cytoskeleton during the life cycle of plant cells. Although microtubules within each array are apparently identical in structure, plants have distinct gene sets coding for both a and b -tubulin (Guo et al., 2009). Tubulin genes are not expressed uniformly during plant development; for example, a particular gene is expressed almost exclusively in reproductive organs (Yu et al., 2009). At cellular levels, distinct a-tubulin genes can be specifically expressed in cells that exit from mitosis when transverse microtubules determine the cell shape (Schroder et al., 2001). Generally, the expression of tubulin isotypes seems to be tissue-specific, a model that is also supported by immunological approaches (Parrotta et al., 2009). The use of different tubulin isoforms is further complicated by mechanisms of post-translational modifications which are used to label subpopulations of microtubules, and that work individually or in combination at the level of single cells to control specific microtubule functions in particular cell domains. The detyrosination/ tyrosination of tubulin is probably involved in controlling the binding of plus-end tracking proteins and motor proteins with microtubule depolymerizing activity (Peris et al., 2009); glutamylation and glycylation are hypothetically involved in the mechanism of microtubule severing by katanin (Sharma et al., 2007). On the other hand, acetylation is a posttranslational modification detected in stable microtubules of most cells, but is also likely to be involved in regulating kinesin-based motility (Gardiner et al., 2007). Recently, the discovery of phosphorylated tobacco tubulin suggested that tyrosine phosphorylation is also involved in regulating the properties of plant microtubules (Blume et al., 2008). A recent report suggests that addition of putrescine to tubulin by pollen transglutaminase can also regulate the binding and release of kinesin to/from microtubules (Del Duca et al., 2009). Consequently, current data from genetic and biochemical approaches suggest a model in which development of specific plant cells and tissues is characterized by the expression of distinct tubulin genes and, consequently, by the use of distinct tubulin isotypes, which are post-translationally modified to control the binding of microtubule-associated proteins (MAPs). MAPs are used to assemble different microtubule arrays according to the specific stage of the cell cycle and their interaction with microtubules is critical at almost every stage of microtubule life. After one microtubule is assembled by a cTuC nucleating complex (containing c-tubulin and several gamma complex proteins or GCPs), stabilization of the growing end is supported by binding to specific proteins, such as MOR1, and/or by putative association with the