Electrostatic attraction between opposite charges is a major force that holds interacting biomolecules together in cells. Traditionally, it was believed that direct contact between biomolecules was required since their surfaces are constantly shielded by ions.

However, according to a new study published in Nature on April 1, researchers from the Shanghai Institute of Organic Chemistry of the Chinese Academy of Sciences have discovered for the first time that the electric dipole moment (EDM) of proteins can drive the dynamics of an important transmembrane signaling pathway through long-range interaction.

The study focuses on tumor necrosis factor receptor-1 (TNFR1), a pro-inflammatory transmembrane receptor with multifaceted functions ranging from inflammation and the innate immune response to cell death. The outcome of TNFR1 signaling is determined by a complex beneath the cell membrane triggered by the engagement of the extracellular ligand.

This so-called "Complex-I" is a transient signalosome with dynamic assembly and disassembly that determines the signaling strength of downstream signaling pathways and cell fate. Complex-I dynamics are manifested by the rapid recruitment of two intracellular components, TRADD and RIPK1, within minutes, followed by the rapid dissociation of RIPK1.

Despite the tremendous biological importance of the dynamic nature of Complex-I, the driving force behind its dynamics has been a long-standing enigma in receptor biology for decades.

Using cryo-electron microscopy, the researchers resolved the high-resolution structure of Complex-I consisting of TNFR1, TRADD and RIPK1. The structure reveals a helical super-complex assembled by 31 copies of death domains (DDs): a layer of TRADD-DD pentamers is sandwiched between layers of TNFR1-DD and RIPK1-DD pentamers.

Additionally, they discovered that the distinctive assembly process is influenced by the specific interaction between three types of DDs, which occur at surfaces with highly complementary electrostatics. The proteins also exhibit strongly polarized surface charge distributions, which give rise to a strong electric dipole moment (EDM) in this helical super-complex.

Interestingly, the EDM direction of the RIPK1 layers is opposite that of the TNFR1 and TRADD layers. In physics, opposing EDMs can cause long-range repulsion between molecules without physical contact. Thus, the researchers proposed that long-range EDM interactions between RIPK1-DD and TNFR1-DD/TRADD-DD would destabilize the DD supercomplex and drive Complex-I dynamics.

Using a series of carefully designed RIPK1 mutants that only altered the EDM without affecting the DD-interacting surface, the researchers proved that increasing the EDM of RIPK1-DD shortens the lifetime of RIPK1 in Complex I and reduces the signaling strength of the NF-kB pathway.

Together, these findings reveal that EDM interactions between proteins drive the dynamics and function of an important signaling complex. They also provide the first direct experimental evidence that electric dipole interactions can play a decisive role in cellular signaling. This opens new perspectives on how biomolecular assemblies are controlled in living systems.

Mechanism for the dynamic assembly and disassembly of TNFR1 signaling Complex-I. (a) A helical super-complex consists of death domains of TNFR1, TRADD and RIPK1. (b) The force between electric dipole moments drives the dynamics of Complex-I. (Image by LIU Jianping)