The protein that shapes bananas, rice grains, and cancer cells turns out to look like a pitchfork

University of California, Davis.

The curve of a banana and the elongation of a cotton fiber have something in common with the division of a human cancer cell. All three depend on the same family of proteins organizing the same type of intracellular structure. A team at the University of California, Davis has now revealed, in atomic-level detail, what that protein looks like. It resembles a pitchfork.

The protein complex is called augmin, and its job is to help branch microtubules, the hollow protein rods that form the cell's internal skeleton. In dividing cells, microtubules assemble into a spindle that pulls chromosomes apart. In growing plant cells, they form a scaffold that guides the expansion of the rigid cellulose wall. When augmin malfunctions, cells divide incorrectly, fail to grow properly, or die.

A structure no one had seen

Scientists discovered in 2007 that animal cells could not form a functioning spindle without augmin. But most researchers assumed the complex was absent from plants. In 2011, Bo Liu, a professor of plant biology at UC Davis, proved them wrong by identifying eight augmin genes in Arabidopsis thaliana, a model plant in the mustard family. The plant version turned out to be structurally similar to its animal counterpart.

What neither side of the plant-animal divide knew was what augmin actually looked like at the molecular level, or how it physically initiates the branching of microtubules. Liu collaborated with Jawdat Al-Bassam, an associate professor of molecular and cellular biology in the same building, to find out.

Md Ashaduzzaman, a postdoctoral fellow in Al-Bassam's lab, cooled purified plant augmin proteins to -196 degrees Celsius and collected thousands of electron microscope images using cryo-EM. He and Al-Bassam spent months assembling the data into a coherent three-dimensional structure.

"Augmin turns out to look like a pitchfork," Al-Bassam said. The team identified the specific structural features the complex uses to attach to existing microtubules and initiate new branches.

From cell skeleton to crop shape

Liu's earlier work had shown that plant augmin does more than assist cell division. It also regulates cell shape. As a plant cell grows, its microtubule scaffold expands and positions enzymes at the growing edges, directing the construction of the cellulose wall. When Liu reduced augmin production in plant cells, the scaffold became disorganized and cell shape went awry.

This has practical agricultural implications. The juice sacs in oranges are giant cells whose dramatic swelling is driven by internal microtubule expansion. Long-grained rice owes its shape to microtubule scaffolds that cause individual cells to elongate. Cotton fiber cells start the size of a red blood cell and then extend thousands-fold, driven by telescoping microtubules.

"It's pretty dramatic," Liu said of the cotton fiber elongation. Understanding how augmin controls this process could help breeders develop crop varieties with specific cell-shape traits, whether that means longer cotton fibers, different grain shapes, or particular fruit characteristics.

The cancer connection

Augmin's relevance extends to human medicine. In humans, altered levels of augmin protein are associated with worse prognosis in certain cancers of the liver, brain, and other organs. The protein complex is also linked to fertility: augmin defects can cause infertility by disrupting the spindle machinery that separates chromosomes during egg and sperm cell formation.

"Some augmin subunits are highly expressed in human cancer cells," Liu noted. Understanding the structural differences between plant and animal augmin, now visible for the first time, could clarify how the complex functions and malfunctions in human disease.

What the structure does not yet tell us

The cryo-EM structure is of plant augmin specifically. While the overall architecture is expected to be similar in the animal version, the detailed structural comparison has not yet been completed. How the structural features identified in this study map onto augmin's behavior in human cancer cells or during human chromosome segregation will require additional structural work on the animal complex.

The structure is also static. Augmin functions dynamically, binding to moving microtubules, recruiting additional proteins, and initiating branching events that happen in real time inside living cells. Connecting the static structural information to the dynamic biology is the next challenge.

"This was a very long endeavor," Al-Bassam said. "It was a real labor of love that required a lot of people working together."

The work was supported by grants from the National Institutes of Health and the National Science Foundation, and utilized the Biological Electron Microscopy Campus Core at UC Davis.