Biological model system helps us understand the 'absorbing state'
A team of EU-funded German scientists has investigated how fibres made of the muscle protein actin behave when they are transported and cross-linked at the same time. In a new study published in the Proceedings of the National Academy of Sciences, the team explain how they discovered that at a certain point, systems suddenly enter a so-called 'absorbing state', albeit without ceasing to consume energy. Their research was funded in part by the COMPNET ('Dynamics and Self-organisation in Complex Cytoskeletal Networks') project, which has clinched a EUR 1.5 million European Research Council (ERC) grant under the EU's Seventh Framework Programme (FP7).
The laws of absorbing states can be best described by a freight train supplied with sufficient energy riding the rails as far as they go. An absorbing state is a state from which a system cannot escape.
The German team successfully demonstrated how these laws work in nature, too. Their method was to build a simple model system consisting of only three components to study the laws of these so-called absorbing states in muscle proteins. The three components are the muscle protein actin, motor proteins responsible for transport and movement in cells, and fascin molecules that cross-link the actin fibres. Using this simple and easy-to-control model allowed the scientists to investigate the fundamental principles of absorbing states.
The scientists were able to analyse how actin is part of an active system, a system that continuously consumes energy. Although active systems are all around us, in both the simplest machines and the most highly developed creatures, our knowledge and understanding of them remains limited.
In the experiment, millions of biological motor proteins anchored on a glass surface were responsible for transporting the actin fibres. They were the active components in the model system. After adding adenosine triphosphate (ATP), the 'fuel' for the motor proteins, the fibres began to move randomly. Next the researchers added cross-linking molecules to connect the fibres. This led to the formation of ever-larger structures that moved around on the glass surface.
Ultimately, all fibres are incorporated into large structures. However, these structures were no longer able to move freely across the surface, and they became fixed in place and ran in circles — the system was trapped in an absorbing state, since it was in a state from which it could not escape.
Much to the surprise of the scientists, the structures that developed were quite complex. The team report finding a collection of perfectly shaped rings made up of millions of individual fibres that permanently rotated under the influence of the motor proteins.
'The amazing thing is not only the complexity of the structures themselves, but the fact that even such a simple system comprising only three components — fibres, motor proteins and cross-linking molecules — can run into an absorbing state,' says study lead author Volker Schaller from the Institute of Molecular and Cellular Biophysics at Technische Universitaet Muenchen in Germany.
For another of the study authors, Andreas Bausch, the fascinating element in their findings is that although an active system can enter an absorbing state, it continues to consume energy. 'The mesmerising thing about the model system, aside from the fascination evoked by the almost perfect patterns, is a seeming contradiction. For the system, an absorbing state is like a dead-end street: once part of the system walks into the dead end, there is no more escape,' explains Professor Bausch.