Two Decades of Studying Functional Amyloids in Microorganisms

The expanding scope of amyloid signalling

Formation of higher-order supramolecular complexes has emerged as a common principle underlying activity of a number of immune and regulated cell-death signalling pathways in animals, plants and fungi. Some of these signalosomes employ functional amyloid motifs in their assembly process. The description of such systems in fungi finds its origin in earlier studies on a fungal prion termed [Het-s], originally identified as a non-Mendelian cytoplasmic infectious element. Janine Beisson has been a key contributor to such early studies. Recent work on this and related systems offers a more integrated view framing this prion in a broader picture including related signalling systems described in animals. We propose here an auto-commentary centred on three recent studies on amyloid signalling in microbes. Collectively, these studies increase our understanding of fold conservation in functional amyloids and the structural basis of seeding, highlight the relation of fungal amyloid motifs to mammalian RHIM (RIP homotypic interaction motif) and expand the concept of Nod-like receptor-based amyloid signalosomes to the prokaryote reign.

Microbial Prions: Dawn of a New Era

Protein misfolding and aggregation are associated with human diseases and aging. However, microorganisms widely exploit the self-propagating properties of misfolded infectious protein particles, prions, as epigenetic information carriers that drive various phenotypic adaptations and encode molecular information. Microbial prion research has faced a paradigm shift in recent years, with breakthroughs that demonstrate the great functional and structural diversity of these agents. Here, we outline unorthodox examples of microbial prions in yeast and other microorganisms, focusing on their noncanonical functions. We discuss novel molecular mechanisms for the inheritance of conformationally-encoded epigenetic information and the evolutionary advantages they confer. Lastly, in light of recent advancements in the field of molecular self-assembly, we present a hypothesis regarding the existence of non-proteinaceous prion-like entities.