A nucleation barrier spring-loads the CBM signalosome for binary activation

Immune cells activate in binary, switch-like fashion via large protein assemblies known as signalosomes, but the molecular mechanism of the switch is not yet understood. Here, we employed an in-cell biophysical approach to dissect the assembly mechanism of the CARD-BCL10-MALT1 (CBM) signalosome, which governs nuclear transcription factor-kappa B activation in both innate and adaptive immunity. We found that the switch consists of a sequence-encoded and deeply conserved nucleation barrier to ordered polymerization by the adaptor protein BCL10. The particular structure of the BCL10 polymers did not matter for activity. Using optogenetic tools and single-cell transcriptional reporters, we discovered that endogenous BCL10 is functionally supersaturated even in unstimulated human cells, and this results in a predetermined response to stimulation upon nucleation by activated CARD multimers. Our findings may inform on the progressive nature of age-associated inflammation, and suggest that signalosome structure has evolved via selection for kinetic rather than equilibrium properties of the proteins.

Automated summary

This study investigates the molecular mechanism of immune cell activation and the assembly mechanism of the signalosome. The researchers discovered that the switch-like activation of immune cells is achieved through large protein assemblies known as signalosomes. They also found that the assembly of the signalosome involves a nucleation barrier and that the structure of the proteins in the signalosome is not critical for its activity.