The Plant TPX2 Protein Regulates Prospindle Assembly before Nuclear Envelope Breakdown

Abstract

The Targeting Protein for Xklp2 (TPX2) is a central regulator of spindle assembly in vertebrate cells. The absence or excess of TPX2 inhibits spindle formation. We have defined a TPX2 signature motif that is present once in vertebrate sequences but twice in plants. Plant TPX2 is predominantly nuclear during interphase and is actively exported before nuclear envelope breakdown to initiate prospindle assembly. It localizes to the spindle microtubules but not to the interdigitating polar microtubules during anaphase or to the phragmoplast as it is rapidly degraded during telophase. We characterized the Arabidopsis thaliana TPX2-targeting domains and show that the protein is able to rescue microtubule assembly in TPX2-depleted Xenopus laevis egg extracts. Injection of antibodies to TPX2 into living plant cells inhibits the onset of mitosis. These results demonstrate that plant TPX2 already functions before nuclear envelope breakdown. Thus, plants have adapted nuclear-cytoplasmic shuttling of TPX2 to maintain proper spindle assembly without centrosomes.

SIDECAR POLLENsuggests a plant-specific regulatory network underlying asymmetric microspore division in Arabidopsis

“…We hypothesize that SCP may trigger a regulatory network for mitotic orientation even before nuclear migration to supply downstream factors directly controlling spindle orientation. Similar to other cases suggesting that spindle positioning and orientation are coupled to the cell cycle, 18,19 delayed mitotic entry observed in scp microspores is not surprising. In addition to the unique features of asymmetric microspore division-with no PPB and the lack of conserved metazoan polarity determinants-the fact that SCP is a microspore-specific LBD/ ASL family protein suggests plant-specific or even microspore-specific mechanisms regulating asymmetric cell division.…”

TPX2 in human clear cell renal carcinoma: Expression, function and prognostic significance

“…The mice were randomly divided into the following two groups: TPX2 vector group and control group. The RCC cells (4x10 7 ) were seeded into the back region of each mouse via subcutaneous injection. All of the mice were observed, with observations recorded for 5 continuous weeks and the size of the xenograft measured once a week (calculated using the formula v = ab 2 π / 6, where a is the longest diameter and b is the longest diameter perpendicular to the tumor).…”

“…Targeting protein for Xenopus kinesin-like protein 2 (TPX2) is a microtubule-associated protein, and functions to regulate the construction of the mitotic spindle by promoting microtubule nucleation from chromatin and stabilization of the spindle microtubules (7,8). It has been demonstrated that TPX2 overexpression induces the amplification of centrosomes and results in DNA polyploidy (9).…”

Mechanisms of plant spindle formation

“…Homologs of NuMA, TPX2, and Rae1 have been identified based on sequence homology and their conserved functions during spindle formation. Similar to its vertebrate homolog, Arabidopsis TPX2 localizes to nuclei during interphase and associates with spindle microtubules during mitosis (Vos et al 2008). AtTPX2 binds importin α and promotes spindle assembly in vitro (Vos et al 2008) and is therefore believed to be involved in the RanGTP-regulated spindle assembly pathway.…”

“…Similar to its vertebrate homolog, Arabidopsis TPX2 localizes to nuclei during interphase and associates with spindle microtubules during mitosis (Vos et al 2008). AtTPX2 binds importin α and promotes spindle assembly in vitro (Vos et al 2008) and is therefore believed to be involved in the RanGTP-regulated spindle assembly pathway. Microinjection of TPX2 antibodies immediately after nuclear envelope breakdown causes cell cycle arrest, suggesting that TPX2 is required to initiate plant spindle assembly (but, interestingly, is not required to complete mitosis (Vos et al 2008)).…”