Alternative fuel for string-shaped motors in cells

Newswise — Cells possess an intriguing ability to neatly arrange their interior through the use of tiny protein machines known as molecular motors that produce targeted movements. The majority of these motors utilize a prevalent form of chemical energy, called ATP, to function. However, a group of researchers from the Max Planck Institute of Molecular Cell Biology and Genetics (MPI-CBG), the Cluster of Excellence Physics of Life (PoL), the Biotechnology Center (BIOTEC) of the TU Dresden in Dresden, Germany, and the National Centre for Biological Sciences (NCBS) in Bangalore, India, have recently identified a fresh molecular system that operates on an alternative chemical energy and employs a unique mechanism to execute mechanical work. This molecular motor, which contracts and expands repeatedly, operates similarly to a classical Stirling engine and assists in the distribution of cargo to membrane-bound organelles. It is the first motor that employs two components, Rab5 and EEA1, which are of different sizes, and is driven by GTP instead of ATP. The findings have been published in the journal Nature Physics.

Motor proteins are exceptional molecular machines found within cells that transform chemical energy, kept in ATP molecules, into mechanical work. The most prominent instance is myosin, which aids in the movement of our muscles. Conversely, small proteins known as GTPases have not been considered as molecular force generators. However, in 2016, an interdisciplinary team of biophysicists and cell biologists led by MPI-CBG directors Marino Zerial and Stephan Grill, as well as their colleagues from PoL and BIOTEC, and research group leader Marcus Jahnel, discovered a molecular motor composed of two proteins, EEA1 and Rab5. They found that the small GTPase protein Rab5 could initiate a contraction in EEA1. These tether proteins in the form of strings can recognize the Rab5 protein present in a vesicle membrane and bind to it. The binding of the much smaller Rab5 sends a message along the extended structure of EEA1, increasing its flexibility, much like how cooking softens spaghetti. This flexibility change generates a force that pulls the vesicle towards the target membrane, leading to docking and fusion. Nevertheless, the team also proposed that EEA1 could switch between a rigid and a flexible state, much like the motion of a mechanical motor, merely by interacting with Rab5 alone.

The present research builds upon the doctoral work of the two first authors of the study, Joan Antoni Soler from Marino Zerial's research group at MPI-CBG and Anupam Singh from the group of Shashi Thutupalli, a biophysicist at the Simons Centre for the Study of Living Machines at the NCBS in Bangalore. Their aim was to experimentally observe this motor in action.

Anupam Singh developed an experimental design to investigate the dynamics of the EEA1 protein, and in 2019, he spent three months at MPI-CBG to conduct the experiments. Anupam explains, "When I met Joan, I explained to him the idea of measuring the protein dynamics of EEA1. But these experiments required specific modifications to the protein that allowed measurement of its flexibility based on its structural changes." Joan Antoni Soler, with his expertise in protein biochemistry, was well-suited to tackle this difficult task. Joan Antoni adds, "I was excited to learn that the approach to characterize the EEA1 protein could answer whether EEA1 and Rab5 form a two-component motor, as previously suspected. I realized that the challenges of obtaining the right molecules could be resolved by modifying the EEA1 protein to allow fluorophores to attach to specific regions of the protein. This modification would make it easier to characterize the protein structure and the changes that can occur when it interacts with Rab5."

Using modified protein molecules and advanced laser scanning microscopes, Joan Antoni Soler and Anupam Singh were able to thoroughly characterize the dynamics of EEA1, with the help of Janelle Lauer, a senior postdoctoral researcher in Marino Zerial's research group. They discovered that EEA1 can undergo multiple flexibility transition cycles, driven solely by the chemical energy released by its interaction with the GTPase Rab5, indicating that EEA1 and Rab5 form a GTP-driven two-component motor. To interpret the results, Marcus Jahnel developed a new physical model that describes the coupling between chemical and mechanical steps in the motor cycle, and together with Stephan Grill and Shashi Thutupalli, they calculated the thermodynamic efficiency of the new motor system, which is comparable to that of conventional ATP-driven motor proteins.

The discovery of the EEA1-Rab5 motor as a new class of molecular motors is groundbreaking, as it challenges the traditional view of small GTPases solely as signaling molecules. The study's results suggest that this two-component motor system is capable of active mechanical roles in membrane trafficking, and the mechanism may be conserved across other molecules and cellular compartments. Moreover, this motor can generate forces and perform work while staying in place, which is different from other molecular motors that move around. The thermodynamic efficiency of this motor system is also comparable to that of conventional ATP-driven motor proteins. These findings open up new possibilities for understanding the mechanisms of cellular processes and could have implications for the development of new therapies for diseases related to cellular transport.

That's an exciting possibility! Synthetic protein engines could have a wide range of applications, from nanotechnology to biomedicine. However, it's worth noting that developing such engines would require a deep understanding of the molecular mechanisms involved, as well as significant advances in protein engineering and synthesis technologies. Nonetheless, the discovery of the EEA1-Rab5 motor system is a significant step forward in this direction.

In general, the authors aspire that this novel interdisciplinary investigation may initiate innovative research paths in both molecular cell biology and biophysics.

Original Publication
Anupam Singh, Joan Antoni Soler, Janelle Lauer, Stephan W Grill, Marcus Jahnel, Marino Zerial, and Shashi Thutupalli: Two-component molecular motor driven by a GTPase cycle. Nature Physics (May 2023)
Link: https://doi.org/10.1038/s41567-023-02009-3