Current Feature
Living matter facilitates exotic behaviors not observed in the physical world that allow for the growth, replication, movement and survival of biological organisms. Biology excels at building hierarchical machines that function at the molecular, cellular and organismal length scales to execute these tasks. Understanding the physical principles underlying these machines presents an exciting challenge at the interface between condensed matter physics and biology.
One important question in this emerging field is how collections of proteins self-assemble into larger length scale structures that effect force transmission and shape change at cellular and tissue length scale. For example, muscle contraction works by myosin motor proteins generating forces on highly organized arrays of actin filaments into striated myofibrils. However, very little is known about how these structures are formed or, to what extent, the precise organization of striated myofibrils is necessary.
The research group led by Margaret Gardel has shed light on this process by creating artificial muscle fibers formed by mixing together solutions of purified actin filaments and myosin motors. Todd Thoresen, a postdoc in the group, found that these bundles where highly disordered but yielded qualitatively similar behaviors to that found in muscle cells. These results were published by the Biophysical Journal. By working with theorists, Martin Lenz and Professor Aaron Dinner, an understanding how disordered collections of motors and actin filaments could yield a net contractile structure emerged. A dispersion of motors speeds generates stresses on filaments within the bundles, resulting in some segments under compression and others under extension. Long, slender rods respond asymmetrically to these stresses, resisting extension while buckling easily under compression. The buckling relieves compressive stresses and results in an overall tensile forces within the structure. This facilitates an overall contractile structure that is robust to disorder and polarity reversal, but quite sensitive to the magnitude of stress generated and the ease of filament buckling. This collaboration between experiment and theory has yielded new insight into how collections of motor proteins and biopolymers can result in contractile forces that are transmitted in networks and bundles to the cellular length scale. These results provide insight into the types of design principles harnessed by biological proteins in building materials used by living matter.
