MED15 prion-like domain forms a coiled-coil responsible for its amyloid conversion and propagation

Abstract

A disordered to β-sheet transition was thought to drive the functional switch of Q/N-rich prions, similar to pathogenic amyloids. However, recent evidence indicates a critical role for coiled-coil (CC) regions within yeast prion domains in amyloid formation. We show that many human prion-like domains (PrLDs) contain CC regions that overlap with polyQ tracts. Most of the proteins bearing these domains are transcriptional coactivators, including the Mediator complex subunit 15 (MED15) involved in bridging enhancers and promoters. We demonstrate that the human MED15-PrLD forms homodimers in solution sustained by CC interactions and that it is this CC fold that mediates the transition towards a β-sheet amyloid state, its chemical or genetic disruption abolishing aggregation. As in functional yeast prions, a GFP globular domain adjacent to MED15-PrLD retains its structural integrity in the amyloid state. Expression of MED15-PrLD in human cells promotes the formation of cytoplasmic and perinuclear inclusions, kidnapping endogenous full-length MED15 to these aggregates in a prion-like manner. The prion-like properties of MED15 are conserved, suggesting novel mechanisms for the function and malfunction of this transcription coactivator.

Decoding the role of coiled-coil motifs in human prion-like proteins

“…However, almost forty years after this seminal discovery, increasing evidence indicates that prion-like conformational conversion is not always pathogenic. On the contrary, it can be exploited for functional roles [ 3 ]. The best-characterized examples of non-pathogenic prions are those identified in yeast and filamentous fungi.…”

“…A less recognized feature of PrDs is the presence of protein sequence stretches with a propensity to form coiled-coils (CCs), often overlapping with the Q/N-rich or polyQ regions [ 3 , 7 ]. This super secondary structure has been associated with the formation of dimers, trimers, and higher-order oligomeric structures.…”

“…This super secondary structure has been associated with the formation of dimers, trimers, and higher-order oligomeric structures. Furthermore, it has been proposed that coiled-coil (CC) formation is an essential step preceding the physiological conformational switches to amyloid-like states in yeast prions [ 3 , 7 ].…”

“…Our group recently exploited different bioinformatics tools to systematically analyse the presence of prion-like domains (PrLDs) at the proteome level [ 2 , 3 , 8 ]. Using this approach, we identified a subset of human polypeptides that bear PrLDs with properties resembling those of yeast PrDs [ 3 , 8 ].…”

“…Our group recently exploited different bioinformatics tools to systematically analyse the presence of prion-like domains (PrLDs) at the proteome level [ 2 , 3 , 8 ]. Using this approach, we identified a subset of human polypeptides that bear PrLDs with properties resembling those of yeast PrDs [ 3 , 8 ]. Subsequently, we explored whether these human domains contain regions with the propensity to fold into CCs using the COILS [ 9 ] and PARCOIL2 [ 10 ] algorithms.…”