Visualizing 'unfurling' microtubule growth

November 13, 2018

Living cells depend absolutely on tubulin, a protein that forms hollow tube-like polymers, called microtubules, that form scaffolding for moving materials inside the cell. Tubulin-based microtubule scaffolding allows cells to move, keeps things in place or moves them around. When cells divide, microtubule fibers pull the chromosomes apart into new cells. Cells with defects in tubulin polymerization die.

Microtubule fibers are hollow rods made of much smaller tubulin subunits that spontaneously assemble at one end of the rod, but exactly how they do this inside the crowded environment of living cells has been a mystery. Now researchers at UC Davis have uncovered the mechanism that puts these blocks in place, illustrated in a new animation.

"It's going to transform how people think about microtubule polymerization," said Jawdat Al-Bassam, associate professor of molecular and cellular biology in the UC Davis College of Biological Sciences. A paper describing the work appears Nov. 13 in the journal eLife.

The work describes snapshots of a set of domains called TOGs, or Tumor Overexpressed Genes, caught in the act of driving tubulin polymerization. As the name suggests, TOGs are abundant in rapidly-dividing cancer cells. They show a similar structure in organisms from yeast to people.

Working in yeast, project scientist Stanley Nithianantham, Al-Bassam and colleagues showed how a protein called Alp14, with four TOG domains, speeds up tubulin polymerization into microtubules by carrying four tubulin units to the correct end of a microtubule and neatly unloads them in the right order to build out the end.

Alp14 represents a group of well-preserved proteins that are essential for cellular homeostasis and division of cells found from a yeast to human cell. It consists of an assembly linked flexible linker with two TOG1 and two TOG2 domains. Add four tubulin units (two per TOG domain) and it forms a circle with the TOGs facing each other and tubulins on the outside.

When the TOG/tubulin circles reach the growing end of a microtubule, TOG1 docks its tubulin with the growing end, destabilizing the circle so that it unfurls, placing four tubulins in order on the end. The name was chosen because the process is like unfurling a folded sail on boat in the wind.

"It's a complete surprise that it's such an ordered, concerted process," Al-Bassam said.

As tubulins units are added to the microtubule strand, they straighten out, driving further disassociation of tubulins from TOGs. The process explains how multiple TOGs speed up tubulin assembly for the first time.

The researchers are following this work with studies of mutant proteins of Alp14 designed with predicted defects in this process to test this suggested mechanism using imaging approaches of dynamic tubulin assembly in and outside living cells. The researchers plan to follow up with further studies of the process, including using cryoelectron microscopy that allows them to visualize single protein molecules in their natural state.
Additional authors on the paper are Brian Cook, Madeleine Beans and Fei Guo at UC Davis and Fred Chang at UC San Francisco. The animation was produced by Julian Eskin of Brandeis University. The work was supported by grants from the NIH and made use of the Advanced Photon Source, a facility operated by the Argonne National Laboratory on behalf of the U.S. Department of Energy Office of Science.

University of California - Davis

Related Microtubules Articles from Brightsurf:

Unbalanced microtubule networks launch establishment of neuronal polarity
Prof. MENG Wenxiang's group from the Institute of Genetics and Developmental Biology of the Chinese Academy of Sciences recently reported a new mechanism by which microtubule networks instruct neuronal polarity.

Biologists unravel tangled mystery of plant cell growth
When cells don't divide into proper copies of themselves, living things fail to grow as they should.

Cellular train track deformities shed light on neurological disease
A new technique allows researchers to test how the deformation of tiny train track-like cell proteins affects their function.

Parkinson's disease protein structure solved inside cells using novel technique
The top contributor to familial Parkinson's disease is mutations in leucine-rich repeat kinase 2 (LRRK2), whose large and difficult structure has finally been solved, paving the way for targeted therapies.

POSTECH developed self-assembled artificial microtubule like LEGO building blocks
Professor Kimoon Kim and his research team identified a new hierarchical self-assembly mechanism

How cells assemble their skeleton
Microtubules, filamentous structures within the cell, are required for many important processes, including cell division and intracellular transport.

Researchers unlock secrets of cell division, define role for protein elevated in cancer
Researchers at Princeton University have successfully recreated a key process involved in cell division in a test tube, uncovering the vital role played by a protein that is elevated in over 25% of all cancers.

Computer model described the dynamic instability of microtubules
Researchers of Sechenov University together with their colleagues from several Russian institutes studied the dynamics of microtubules that form the basis of the cytoskeleton and take part in the transfer of particles within a cell and its division.

A simple way to control swarming molecular machines
The swarming behavior of about 100 million molecular machines can be controlled by applying simple mechanical stimuli such as extension and contraction.

Cancer tumours form surprising connections with healthy brain cells
Anti-epileptic medicine can curb the dangerous communication and possibly be part of future treatment.

Read More: Microtubules News and Microtubules Current Events is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to