How proteins form spatial gradients without any sources or sinks

The steady spatial gradients of different proteins are one of the most important mechanisms cells use to transmit information within their volume, but scientists have yet to understand the process of how certain gradients are formed, particularly for those proteins without any sources or sinks. These proteins are crucial to a number of biological processes, such as the germline formation in C. elegans or spindle assembly.

To better understand the physics behind these biological processes, researchers have proposed a novel generic mechanism of how proteins without any sources or sinks form gradients, as reported in a new article published in The Journal of Chemical Physics. The authors find that these proteins can form gradients if they are coupled to other molecules (dubbed “pronogens”) that act within the same cell, and presented a theoretical model that shows how a pronogen that diffuses within a single cell can interact with proteins to form both pronogen and protein steady-state gradients.

Pronogens, defined as small diffusing molecules, behave similarly to morphogens in multicellular assemblies. They have a localized source at one end of the cell with uniform degradation throughout the system, which results in a steady-state concentration gradient. Proteins coupled to the pronogens can then also develop a concentration gradient. By solving nonlinear differential equations with certain boundary conditions, they demonstrate the feasibility of such a process in biological systems.

Through analysis, they discovered that the gradients of proteins may follow or be complementary to that of the pronogen gradients, depending on the nature of protein-pronogen interaction. They also found that the process is linearly stable with respect to external perturbations. The authors hope their work inspires experimentalists to explore this mechanism in more detail.

Source: “Protein gradients in single cells induced by their coupling to ‘morphogen’-like diffusion,” by Saroj Kumar Nandi and Sam A. Safran, The Journal of Chemical Physics (2018). The article can be accessed at https://doi.org/10.1063/1.5021086 .