(Nanowerk News) Cells have the interesting feature of organizing their interiors neatly using tiny protein machines called molecular motors that produce directional movements. Most of them use a common type of fuel, a type of chemical energy, called ATP to operate.
Now researchers from the Max Planck Institute of Molecular Cell Biology and Genetics (MPI-CBG), the Cluster of Excellence Physics of Life (PoL) and the Biotechnology Center (BIOTEC) of TU Dresden in Dresden, Germany, and the National Center for Biological Sciences (NCBS) ) in Bangalore, India, discovered a new molecular system that uses alternative chemical energy and uses a new mechanism to do mechanical work.
By repeatedly contracting and expanding, these molecular motors function similarly to the classic Stirling engine and help distribute loads to membrane-bound organelles. It is the first motor to use two components, two different sized proteins, Rab5 and EEA1, and is driven by GTP instead of ATP.
The results are published in a journal Natural Physics (“A two-component molecular motor driven by the GTPases cycle”).
Motor proteins are extraordinary molecular machines in cells that convert chemical energy, which is stored in a molecule called ATP, into mechanical work. The most prominent example is myosin which helps our muscles to move. In contrast, GTPases, which are small proteins, have not been viewed as generators of molecular forces.
One example is the molecular motor which consists of two proteins, EEA1 and Rab5. In 2016, an interdisciplinary team of cell biologists and biophysicists in the group of MPI-CBG directors Marino Zerial and Stephan Grill and their colleagues, including PoL and BIOTEC research group leader Marcus Jahnel, discovered that the small GTPase protein Rab5 can trigger contraction in EEA1. This strap-shaped binding protein can recognize the Rab5 protein present in the vesicle membrane and bind to it. The much smaller binding of Rab5 sends messages along the elongated structure of EEA1, thereby increasing its malleability, similar to the way cooking tenderizes spaghetti.
Such a change in flexibility generates a force that pulls the vesicle toward the target membrane, where docking and fusion occur. However, the team also hypothesized that EEA1 could switch between flexible and rigid states, similar to mechanical motor movement, by interacting with Rab5 alone.
This is where the current research began, shaped by the doctoral work of the study’s first two authors. Joan Antoni Soler of Marino Zerial’s research group at MPI-CBG and Anupam Singh of Shashi Thutupalli’s group, biophysicists at the Simons Center for the Study of Living Machinery at NCBS in Bangalore, set out to experimentally observe this motor in action.
With an experimental design to investigate EEA1 protein dynamics, Anupam Singh spent three months at MPI-CBG in 2019. “When I met Joan, I explained to her the idea of measuring EEA1 protein dynamics. based on its structural changes,” said Anupam. Joan Antoni Soler’s expertise in protein biochemistry was a perfect fit for this challenging task.
“I was pleased to learn that this approach to characterizing the EEA1 protein could answer whether EEA1 and Rab5 form a two-component motor, as previously suspected. I realized that the difficulty in obtaining the right molecule could be solved by modifying the EEA1 protein to allow the fluorophore to attach to a specific protein region. This modification will make it easier to characterize the structure of the protein and the changes that can occur when it interacts with Rab5, “explained Joan Antoni.
Armed with suitable protein molecules and the valuable support of co-author Janelle Lauer, a senior postdoctoral researcher in Marino Zerial’s research group, Joan and Anupam were able to thoroughly characterize EEA1 dynamics using state-of-the-art laser scanning microscopy provided by the light microscopy facility. in MPI-CBG and NCBS. Surprisingly, they found that the EEA1 protein can undergo multiple cycles of flexibility transitions, from rigid to flexible and back again, driven solely by the chemical energy released by its interaction with the GTPase Rab5. These experiments demonstrate that EEA1 and Rab5 form a GTP-driven two-component motor.
To interpret the results, Marcus Jahnel, co-author of the correspondence and leader of the research group at PoL and BIOTEC, developed a new physical model to describe the relationship between chemical and mechanical steps in motorcycles. Together with Stephan Grill and Shashi Thutupalli, the biophysicists were also able to calculate the thermodynamic efficiency of the new motor system, which is comparable to that of conventional ATP-driven motor proteins.
“Our results show that the EEA1 and Rab5 proteins work together as a two-component molecular motor system that can transfer chemical energy into mechanical work. Consequently, they can play an active mechanical role in membrane trafficking. It is possible that the power-generating molecular motor mechanisms may be conserved in other molecules and used by some other cellular compartments,” Marino Zerial summarized the study.
Marcus Jahnel added: “I’m excited that we can finally test the EEA1-Rab5 motor idea. It’s great to see it confirmed by this new experiment. Most molecular motors use a common type of cellular fuel called ATP. Small GTPases consume other types of fuel, GTP, and have been considered primarily as signaling molecules. That they can also drive molecular systems to generate forces and move objects puts these abundant molecules in an exciting new light.”
Stephan Grill is equally excited: “This is a new class of molecular motors! They don’t move like kinesin motors transporting charges along microtubules, but do work while remaining in place. A bit like octopus tentacles.”
“The model we used was inspired by the classic Stirling engine cycle. While the traditional Stirling engine produces mechanical work by expanding and compressing the gas, the two-component motor described uses protein as the working substrate, with changes in the flexibility of the protein generating force. As a result, this type of mechanism opens up new possibilities for the development of synthetic protein machinery,” added Shashi Thutupalli.
Overall, the authors hope that this new interdisciplinary study can open new research avenues in both molecular cell biology and biophysics.