Research Focus: Molecular motor grows cell's microtubules

Kinesin on microtubules

UNIVERSITY PARK, Pa. -- Motor proteins that pause at the ends of microtubules and produce pushing forces can also stimulate their growth, according to researchers at Penn State. The proteins’ function could be a critical component in understanding cell division and nerve branching and growth.

Kinesins are a family of motor proteins found in multicellular organisms. They function as ‘little engines’ within the cells and transport molecular cargo along microtubules among other things. The microtubules -- 25 nanometers thick or one ten thousandth the diameter of a human hair -- are hollow cylinders of the protein tubulin. They are extremely dynamic and have the ability to grow and shrink as the cell changes shape.

“We are trying to get under the hood of these motors and understand what makes their sequences unique,” said William Hancock, professor of biomedical engineering, Penn State. “Because they carry out so many vital functions in the cells of the body, we really want to understand how they operate at the molecular level.”

In total, there are 45 different kinesin motor proteins.  Hancock and Yalei Chen, graduate student in cell and developmental biology, Penn State's Huck Institute of the Life Sciences, looked at modified kinesin-5 in the laboratory and found that the motor pauses at the end of the microtubules.  It then generates pushing forces against other microtubules in the bundles. The pushing forces slide the microtubules apart and essentially allow the motor to grow the microtubules.  The researchers reported their results in a recent issue of Nature Communications. 

The researchers placed microtubules on a surface and added free tubulin subunits together with modified kinesin-5 motor proteins. The results, observed under a fluorescence microscope, showed that the addition of the motor proteins increased both the rate and the persistence of microtubule growth.

Understanding the proteins’ role in microtubule growth also provided a better understanding of its role in cell division, as microtubules join to form the mitotic spindle.

The mitotic spindle is a scaffold that ensures duplicated chromosomes are properly segregated from the original cell into two new cells. The final result is two completely new cells containing identical genetic material. Understanding how the scaffold is formed is useful in understanding cell division and is also useful in understanding how to inhibit cell division when necessary.

The researchers hope to continue their work and investigate how kinesin-5 influences microtubule dynamics in dividing cells and the role it plays in formation and maintenance of genetic material in cell division.

“Cancer cells are the fastest growing cells in a cell population,” Hancock said. “They are the ones that most need to be able to break down and reassemble their microtubule network.”

The ability to terminate cancer cell division could one day lead to new approaches in treatment.

The National Institute of General Medical Sciences, a division of the National Institutes of Health supported this work.