Microtubule-dependent microtubule nucleation based on recruitment of γ-tubulin in higher plants

Abstract

Despite the absence of a conspicuous microtubule-organizing centre, microtubules in plant cells at interphase are present in the cell cortex as a well oriented array. A recent report suggests that microtubule nucleation sites for the array are capable of associating with and dissociating from the cortex. Here, we show that nucleation requires extant cortical microtubules, onto which cytosolic gamma-tubulin is recruited. In both living cells and the cell-free system, microtubules are nucleated as branches on the extant cortical microtubules. The branch points contain gamma-tubulin, which is abundant in the cytoplasm, and microtubule nucleation in the cell-free system is prevented by inhibiting gamma-tubulin function with a specific antibody. When isolated plasma membrane with microtubules is exposed to purified neuro-tubulin, no microtubules are nucleated. However, when the membrane is exposed to a cytosolic extract, gamma-tubulin binds microtubules on the membrane, and after a subsequent incubation in neuro-tubulin, microtubules are nucleated on the pre-existing microtubules. We propose that a cytoplasmic gamma-tubulin complex shuttles between the cytoplasm and the side of a cortical microtubule, and has nucleation activity only when bound to the microtubule.

Computer simulation and mathematical models of the noncentrosomal plant cortical microtubule cytoskeleton

“…CMTs originate from c-tubulin-containing nucleation complexes that are scattered throughout the cell cortex [Liu et al, 1993;Erhardt et al, 2002;Seltzer et al, 2007;Nakamura et al, 2010]. Some of these nucleation complexes associate with the lateral walls of preexisting CMTs, resulting in branch-form microtubule nucleation [Wasteneys and Williamson, 1989a;Murata et al, 2005;Chan et al, 2009;Nakamura et al, 2010]. The CMTs are released from their nucleation sites by the activity of microtubule-severing enzymes [Nakamura and Hashimoto, 2009;Nakamura et al, 2010].…”

“…Some of these CMTs originate in a microtubule-independent manner while others originate from the sides of existing CMTs. In the latter case, the newly formed CMT grows either at an acute angle to the mother CMT (called branch-form nucleation) or parallel to the mother CMT [Wasteneys andWilliamson, 1989a, 1989b;Murata et al, 2005;Ambrose and Wasteneys, 2008;Chan et al, 2009]. The simulations of Allard et al [2010b] considered only branch-form nucleation and implemented it with and without microtubuleindependent nucleation.…”

Collision induced spatial organization of microtubules

“…The in-vivo observations of the reorientation of cortical microtubules in parallel arrays [22,23,26,27] are also ascribed to dynamic instability: the depolymerization of disordered microtubules is followed by the repolymerization into an ordered array. However, the in-vivo selforganization can be regulated by MAPs or motor proteins [7,28] or it may be a result of the simultaneous action of many factors. In this paper we explore the consequences of the simplest possible physical hypothesis for explaining microtubule orientation from a coupling mechanism between growth and inter-tubule interactions.…”

Structure, function, and evolution of plant NIMA-related kinases: implication for phosphorylation-dependent microtubule regulation

“…In animal cells, γTuRC localizes to the centrosome, which is a conspicuous microtubule organizing center. Although plant cells have no centrosome, the γ-tubulin complexes are localized on the side of microtubules, on the nuclear envelope, and on the plastid surface to initiate microtubule nucleation (Liu et al 1994; Kumagai et al 2003; Shimamura et al 2004;Murata et al 2005). …”

“…The γ-tubulin and γ-tubulin complex proteins are essential for microtubule organization, cell expansion and cell division (Binarová et al 2006;Pastuglia et al 2006;Nakamura and Hashimoto 2009; Zeng et al 2009; Kong et al 2010). The γ-tubulin complex is recruited onto pre-existing microtubules and microtubule nucleation could occur by branching from extant microtubules (Murata et al 2005;Murata and Hasebe 2007;Murata et al 2013). Live cell imaging of the γ-tubulin complex revealed that microtubule nucleation is promoted by the association of the γ-tubulin complex with microtubules and newly formed daughter microtubules are dissected from the nucleation complex probably through the activity of katanin (Nakamura et al 2010).…”