Functional amyloids of eukaryotes: criteria, classification, and biological significance

Amyloids cause incurable diseases in humans and animals and regulate vital processes in bacteria and eukaryotes. Amyloid fibrils have unique properties, such as amazing resistance to a variety of agents, mechanical strength, and elasticity, and it is not surprising that in the course of evolution eukaryotes have learned to employ amyloid structures to regulate various vital processes. Proteins exhibiting amyloid properties have been detected in lower eukaryotes and in diverse cell lines of arthropods and vertebrates. A growing number of studies of eukaryotic proteins that demonstrate certain amyloid-like properties require clear criteria to systematize modern knowledge about the functional amyloids. In this review, we propose to separate eukaryotic proteins, whose amyloid properties are clearly proven, and proteins, which show some amyloid characteristics in vivo or in vitro. In order to assert that a protein is a functional amyloid, it is necessary to prove that it has a cross-beta structure in vivo. Here, we consider the advantages and disadvantages of various methods for the analysis of the amyloid properties of a protein. Analysis of the current data shows that amyloids play an important role in the regulation of vital processes in eukaryotes, and new functional amyloids should be searched primarily among structural, protective, and storage proteins. A systematic search for functional amyloids in eukaryotes is only beginning, and the use of novel proteomic methods opens up great prospects for identification of amyloids in any organs and tissues of various organisms.