Assembly of platforms for signal transduction in the new era: dimerization, helical filament assembly, and beyond

Modeling Necroptotic and Pyroptotic Signaling in Saccharomyces cerevisiae

The yeast Saccharomyces cerevisiae is the paradigm of a eukaryotic model organism. In virtue of a substantial degree of functional conservation, it has been extensively exploited to understand multiple aspects of the genetic, molecular, and cellular biology of human disease. Many aspects of cell signaling in cancer, aging, or metabolic diseases have been tackled in yeast. Here, we review the strategies undertaken throughout the years for the development of humanized yeast models to study regulated cell death (RCD) pathways in general, and specifically, those related to innate immunity and inflammation, with an emphasis on pyroptosis and necroptosis. Such pathways involve the assembly of distinct modular signaling complexes such as the inflammasome and the necrosome. Like other supramolecular organizing centers (SMOCs), such intricate molecular arrangements trigger the activity of enzymes, like caspases or protein kinases, culminating in the activation of lytic pore-forming final effectors, respectively, Gasdermin D (GSDMD) in pyroptosis and MLKL in necroptosis. Even though pathways related to those governing innate immunity and inflammation in mammals are missing in fungi, the heterologous expression of their components in the S. cerevisiae model provides a "cellular test tube" to readily study their properties and interactions, thus constituting a valuable tool for finding novel therapies.

Signalling by co-operative higher-order assembly formation: linking evidence at molecular and cellular levels

The concept of higher-order assembly signalling or signalling by co-operative assembly formation (SCAF) was proposed based on the structures of signalling assemblies formed by proteins featuring domains from the death-fold family and the Toll/interleukin-1 receptor domain family. Because these domains form filamentous assemblies upon stimulation and activate downstream pathways through induced proximity, they were envisioned to sharpen response thresholds through the extreme co-operativity of higher-order assembly. Recent findings demonstrate that a central feature of the SCAF mechanism is the nucleation barrier that allows a switch-like, digital or 'all-or-none' response to minute stimuli. In agreement, this signalling mechanism features in cell-death and innate immunity activation pathways where a binary decision is required. Here, we broaden the concept of SCAF to encapsulate the essential kinetic properties of open-ended assembly in signalling, compare properties of filamentous assemblies and other co-operative assemblies such as biomolecular condensates, and review how this concept operates in cells.