Inside every cell, there is a scaffold of filaments, like construction scaffolding that gives the cell its shape and allows it to move. These filaments are connected by crosslinking proteins: some simply hold them together like ropes, while others act as tiny motors that crawl along the filaments and pull them toward each other. How well this team works together determines whether a cell can divide, crawl, or change shape.

For a long time, there was no unified theory to predict the total force generated by a mixture of passive and motor crosslinkers. The authors of this new work derived such a theory, drawing on the thermodynamics of nonequilibrium systems — a branch of physics that describes processes far from rest. Their formula breaks the total force into three parts: entropic (the drive toward disorder), active (the motion of motors), and frictional (friction). This is like breaking down the effort of a moving crew into each person's contribution: who pulls, who braces, who just gets in the way.

The formula was tested on four experiments that examined how passive and motor crosslinkers together affect pairs of filaments. The theory allowed the researchers to separate the contributions of each of the three forces and identify which physical process is responsible for the total force in each case. This makes it possible to quantitatively compare different combinations of crosslinkers and predict how they will affect the formation of patterns along filaments and the dynamics of filament pairs.

Such a theory is a step toward treating cell behavior as an engineering problem. Knowing the properties of the proteins, one can predict how the cytoskeleton will change if certain crosslinkers are added or removed. This is important for understanding many diseases where cell mechanics are disrupted, and for creating artificial cellular structures.