Nucleophosmin/B23 Inhibits Eg5-mediated Microtubule Depolymerization by Inactivating Its ATPase Activity

Abstract

Nucleophosmin/B23, an abundant nucleolar protein, plays multiple roles in cell growth and proliferation, and yet, little has been studied about its function in regulating dynamics of microtubules. Here, we report that B23 directly interacts with Eg5, a member of the kinesin family, in the cytosol. The DNA/RNA binding domain of B23 and the motor domain of Eg5 were found to be involved in their interaction. Both in vivo and in vitro evidences showed that B23 acts as an upstream regulator of Eg5 in promoting microtubule polymerization. Moreover, we further demonstrated that B23 regulates microtubule dynamics by directly inhibiting Eg5 ATPase activity. Microtubule (MT)3 dynamics exist in two major states, either growing or shrinking, and switch stochastically between these two states. The transition from growth to shrinkage is called a catastrophe, and in contrast, transition from shrinkage to growth is called a rescue (1). Both growing and shrinking processes are regulated by various cellular factors that stabilize and/or destabilize the MT lattice. Eg5 (also known as kinesin-5, KIF11), a motor protein that belongs to the kinesin-like protein family, was recently reported to mediate length-dependent control of MT assembly (2, 3). It is proposed that kinesins disassemble longer kinetochore microtubule by generating a concentration gradient along a MT and hydrolyze ATP to move toward the plus (ϩ) end, where it causes MT depolymerization (4, 5). It is now a well established fact that kinesin-related proteins can destabilize microtubules, and the kinesins that destabilize the ends of microtubules are known as "catastrophe kinesins" (6). However, how catastrophe kinesins are regulated remains to be further investigated. Nucleophosmin (also known as B23) is a multifunctional phosphoprotein that shuttles rapidly between the nucleus and the cytoplasm (7, 8). B23 provokes cell proliferation and transformation, and B23 is frequently mutated in many different tumor types such as acute myeloid leukemia (AML) (9 -11). Despite the fact that its functions inside of the nucleus have been well studied, the functional role of B23 in the cytoplasm remains elusive. Previous studies have shown that B23 regulates centrosome duplication during mitosis (12-14), which hints that the protein B23 also may play roles in the global control of the MT lattice.In this study, we identified that B23/nucleophosmin interacts directly with Eg5, and through this interaction, B23 inhibited ATPase activity of the Eg5 motor domain and thus down-regulated the Eg5-mediated ATP-dependent microtubule catastrophe. The physiological effects of Eg5 on microtubules are more significant upon the elimination of B23 protein levels, indicating that B23 normally plays a protective role in MT assembly. EXPERIMENTAL PROCEDURESCell Culture, Drug Treatments, Transfection, and siRNACells were maintained in Dulbecco's modified Eagle's medium (Invitrogen) containing 10% fetal bovine serum. Cells were transfected by Lipofectamine 2000 (Invitrogen) according to the manufacture...

Suppressor analysis uncovers that MAPs and microtubule dynamics balance with the Cut7/Kinesin-5 motor for mitotic spindle assembly inSchizosaccharomyces pombe

“…These results raise the intriguing question as to whether Kinesin-5 molecules per se play any role in microtubule stability and/or dynamics. Previous reports indeed show that Kinesin-5 regulates microtubule stability/dynamics as a microtubule depolymerizing factor (budding yeast Cin8 and human Eg5/Kif11) (GARDNER et al 2008;WANG et al 2010), conversely as a microtubule stabilizer (budding yeast Kip1) (FRIDMAN et al 2013) or a microtubule polymerase (Xenopus Eg5) (CHEN AND HANCOCK 2015). In fission yeast, mutations in genes encoding the components of -TuC other than Alp16 are also capable of rescuing cut7-22 (RODRIGUEZ et al 2008), and interestingly, Cut7/Kinesin-5 reportedly interacts with -TuC (OLMSTED et al 2014).…”

Kinesin-5: Cross-bridging mechanism to targeted clinical therapy

“…Intriguing hints for a regulatory machinery that may interact with Kinesin-5 and regulate its function is suggested by experiments that isolate Kinesin-5 in complex with a variety of microtubule associated proteins (Gable et al, 2012; Iwakiri et al, 2013; Ma et al, 2011; Uteng et al, 2008; Wang et al, 2010), modified by protein kinases (Garcia et al, 2009; Rapley et al, 2008), or in association with other cell division protein complexes (Tan et al, 2012). The putative functional relationships of these players remains to be determined, although in each case some interplay or synergism with mitotic outcome is apparent.…”

Kinesin-5 is a microtubule polymerase

“…Gardner et al . 25 found that knocking out the yeast kinesin-5 Cin8 resulted in longer kinetochore and astral microtubules, suggesting that the motor acts as a depolymerase 26 . In contrast, Fridman et al .…”