Beyond Amyloid Fibers: Accumulation, Biological Relevance, and Regulation of Higher-Order Prion Architectures

Abstract

The formation of amyloid fibers is associated with a diverse range of disease and phenotypic states. These amyloid fibers often assemble into multi-protofibril, high-order architectures in vivo and in vitro. Prion propagation in yeast, an amyloid-based process, represents an attractive model to explore the link between these aggregation states and the biological consequences of amyloid dynamics. Here, we integrate the current state of knowledge, highlight opportunities for further insight, and draw parallels to more complex systems in vitro. Evidence suggests that high-order fibril architectures are present ex vivo from disease relevant environments and under permissive conditions in vivo in yeast, including but not limited to those leading to prion formation or instability. The biological significance of these latter amyloid architectures or how they may be regulated is, however, complicated by inconsistent experimental conditions and analytical methods, although the Hsp70 chaperone Ssa1/2 is likely involved. Transition between assembly states could form a mechanistic basis to explain some confounding observations surrounding prion regulation but is limited by a lack of unified methodology to biophysically compare these assembly states. Future exciting experimental entryways may offer opportunities for further insight.

Keywords: [PSI+]; amyloid; amyloidosis; fibril; prion; protofibrils.

Figure 1

Amyloid higher-order architecture: an assembly state of unknown significance. Protofibrils are assembled from monomeric amyloidogenic proteins through direct association (arrow 1) of the same domain of the protein (blue) among adjacent monomers. This association results in the conformational conversion of the domain (linear to corkscrew) to form a b-sheet rich amyloid core with the non-amyloid domains of the protein (green circles) arrayed on the surface of the protofibril. These protofibrils can then laterally associate into higher-order fibril architectures (arrow 2). The prevalence of these assembly states or how they influence amyloid biology is poorly understood.

Figure 2

The [PSI⁺] prion propagation cycle in the budding yeast Saccharomyces cerevisiae. Newly synthesized Sup35 (green and blue ball and stick) from translating ribosomes (gray) forms prions via interaction of its prion-determining domain (blue), leading to its conformation conversion (linear to corkscrew) and the display of its translational release domain (green circle) on the surface of the protofibril. Conversion occurs de novo at low frequency when overexpressed (not shown) or through association with existing linear prion complexes (ball and corkscrew wheels) via the prion-determining domain (blue) through templated conversion at particle ends (double arrow), increasing particle size and reducing particle mobility. Fragmentation of particles by the disaggregase Hsp104 (white hexamer of ellipses) in cooperation with the Hsp70/40 (Ssa1/2) chaperone system (not shown) increases particle number, amplifying the abundance of conversion surfaces available for templating, decreasing the size of these particles and increasing their mobility (green and blue ball and corkscrew wheel fragments). Small mobile particles transmit into daughter cells at cell division maintaining the prion in the cell population.

Figure 3

Is [PSI⁺] higher-order architecture controlled by chaperone binding? Hsp70 (Ssa1/2; white) appears to be associated with [PSI⁺] prion complexes (green and blue ball and corkscrew wheel stacks); however, what determines association and disassociation from these complexes or how this impacts protofibril association into higher-order fibril architectures is unclear.