Ran GTPase regulates non-centrosomal microtubule nucleation and is transported by actin waves towards the neurite tip

Yung-An Huang, Chih-Hsuan Hsu, Ho-Chieh Chiu, Chris T. Ho, Wei-Lun Lo, Eric Hwang

Preprint posted on June 28, 2019

On the Ran: the small GTPase Ran promotes non-centrosomal microtubule nucleation in neurons and is transported to the neurite tip by actin waves

Selected by Ramona Jühlen


In neurons microtubules are nucleated from the centrosome, but as neurons mature the centrosome loses its ability to act as a microtubule-organizing center (MTOC). Several cellular components in neurons have been identified to serve as non-centrosomal MTOC (ncMTOC): the Golgi, the augmin complex and the protein TPX2.

The Ras-related nuclear protein (Ran) is a small GTPase predominately regulating nucleocytoplasmic transport, but also mitotic spindle formation. Ran regulates TPX2 activity and thereby, TPX2-mediated ncMT nucleation in mitotic neurons. The question of whether and how Ran plays a role in MT nucleation in post-mitotic neurons remains open. However, there is evidence that Ran regulates neuronal morphogenesis and is enriched at the tip of the neurite.

By using an optogenetic tool the authors shed light on the ability of Ran to nucleate ncMT and discover how Ran is localized to the neurite tip.

Key findings

First, the authors show that specifically activated Ran (RanGTP) is able to regulate MT formation at the neurite tip. For this purpose they used dissociated hippocampal neurons (2 days in vitro) expressing Ran mutants: the constitutively active Ran (RanQ69L; mimicking activated RanGTP) and the dominant negative Ran (RanT24N; reducing RanGTP levels).

In order to verify that RanGTP is able to nucleate MT at other sites in the neuron than the neurite tip, the authors used an optogenetic tool (called RanTRAP; a variation of the original LOVTRAP system (1)) to spatially control RanGTP levels at any location in the neuron. By releasing RanGTP at the photoactivated region, they could verify that RanGTP is indeed able to enhance nucleation of MTs along the neurite.

The authors showed earlier in the preprint that RanGTP is enriched at the neurite tip, but since Ran is potentially produced in the soma of the neuron, they were curious about how Ran is localized or transported to the neurite tip. There has been some evidence that actin-based structures are involved in that process: firstly, RanGTP co-localizes with actin filaments in the growth cone of the neuron and secondly, it has been observed that wave-like dynamics of actin filaments, transporting a variety of protein cargoes in neurons, proceed MT nucleation. By depolymerizing actin filaments the authors present that RanGTP is no longer enriched at the neurite tip and that the frequency of MT nucleation is reduced (leaving MT polymerization velocity unchanged). Finally, they show in live cell imaging that the RanGTP-mimic mutant co-migrates with actin waves, whereas the dominant negative Ran-mutant does not.

Overall, the authors show that Ran plays a crucial role in regulating ncMT nucleation along the neurite and they speculate that RanGTP promotes ncMT nucleation by releasing TPX2 from the inhibitory importin heterodimers. TPX2 can then interact with γ-tubulin and the augmin complex in order to initiate MT nucleation (Figure 1).

Figure 1. Scheme explaining the optogenetic tool RanTRAP and the hypothetical mechanism of RanGTP regulating non-centrosomal microtubule nucleation. The LOV2 domain is fused to a mitochondrial targeting sequence (TOM20) and the RanGTP-mimic RanQ69L is fused to the ZDK domain. In absence of light the activity of RanQ69L is sequestered to the mitochondria (LOV2 binds to ZDK). Upon light irradiation, RanQ69L is released from the mitochondria (LOV2 and ZDK dissociate due to a conformational change in LOV2) and activates TPX2 activity by binding to the inhibitory importin-α/β heterodimer. TPX2 can initiate microtubule nucleation.

What I like about this work

I am fascinated about this kind of protein moonlighting: in my research, so far, I was only concerned about the crucial role of RanGTPase cycling for nucleocytoplasmic import and export. But the obvious LINC between the nucleus and the cytoskeleton, composed of SUN-proteins and nesprins, gets more and more competition by nuclear pore components and associated proteins.

Moreover, this work provides one more and novel indirect connection between the actin and microtubule cytoskeleton.

Open questions

It has been recently shown that CRM1 (exportin 1) can serve as a docking station for ncMT nucleation in yeast (2). How is the current work about Ran and the work by Bao et al. about CRM1 ncMT nucleation possibly linked?

Additional references

1. H. Wang et al., Nat Methods. 13, 755–758 (2016).

2. X. X. Bao et al., eLife. 7, doi:10.7554/eLife.33465.

Tags: actin, lovtrap, non-centrosomal microtubules, rangtpase

Posted on: 16th July 2019


Read preprint (No Ratings Yet)

Have your say

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Sign up to customise the site to your preferences and to receive alerts

Register here

Also in the cell biology category:

FENS 2020

A collection of preprints presented during the virtual meeting of the Federation of European Neuroscience Societies (FENS) in 2020


List by Ana Dorrego-Rivas

Planar Cell Polarity – PCP

This preList contains preprints about the latest findings on Planar Cell Polarity (PCP) in various model organisms at the molecular, cellular and tissue levels.


List by Ana Dorrego-Rivas

BioMalPar XVI: Biology and Pathology of the Malaria Parasite

[under construction] Preprints presented at the (fully virtual) EMBL BioMalPar XVI, 17-18 May 2020 #emblmalaria


List by Dey Lab, Samantha Seah


Cell Polarity

Recent research from the field of cell polarity is summarized in this list of preprints. It comprises of studies focusing on various forms of cell polarity ranging from epithelial polarity, planar cell polarity to front-to-rear polarity.


List by Yamini Ravichandran

TAGC 2020

Preprints recently presented at the virtual Allied Genetics Conference, April 22-26, 2020. #TAGC20


List by Maiko Kitaoka, Madhuja Samaddar, Miguel V. Almeida, Sejal Davla, Jennifer Ann Black, Dey Lab

3D Gastruloids

A curated list of preprints related to Gastruloids (in vitro models of early development obtained by 3D aggregation of embryonic cells). Preprint missing? Don't hesitate to let us know.


List by Paul Gerald L. Sanchez and Stefano Vianello

ECFG15 – Fungal biology

Preprints presented at 15th European Conference on Fungal Genetics 17-20 February 2020 Rome


List by Hiral Shah

ASCB EMBO Annual Meeting 2019

A collection of preprints presented at the 2019 ASCB EMBO Meeting in Washington, DC (December 7-11)


List by Madhuja Samaddar, Ramona Jühlen, Amanda Haage, Laura McCormick, Maiko Kitaoka

EMBL Seeing is Believing – Imaging the Molecular Processes of Life

Preprints discussed at the 2019 edition of Seeing is Believing, at EMBL Heidelberg from the 9th-12th October 2019


List by Dey Lab


Preprints on autophagy and lysosomal degradation and its role in neurodegeneration and disease. Includes molecular mechanisms, upstream signalling and regulation as well as studies on pharmaceutical interventions to upregulate the process.


List by Sandra Malmgren Hill

Lung Disease and Regeneration

This preprint list compiles highlights from the field of lung biology.


List by Rob Hynds

Cellular metabolism

A curated list of preprints related to cellular metabolism at Biorxiv by Pablo Ranea Robles from the Prelights community. Special interest on lipid metabolism, peroxisomes and mitochondria.


List by Pablo Ranea Robles

BSCB/BSDB Annual Meeting 2019

Preprints presented at the BSCB/BSDB Annual Meeting 2019


List by Dey Lab

Biophysical Society Annual Meeting 2019

Few of the preprints that were discussed in the recent BPS annual meeting at Baltimore, USA


List by Joseph Jose Thottacherry

ASCB/EMBO Annual Meeting 2018

This list relates to preprints that were discussed at the recent ASCB conference.


List by Dey Lab, Amanda Haage