Ruolo dei domini coiled coil nel reclutamento dei macchinari molecolari mitocondriali e autofagici in aggregati di proteine con espansioni di polyQ

Polyglutamine (polyQ) expansion diseases, including Huntington disease (HD), are caused by the expansion of coding CAG triplet repeats in certain genes, leading to the production of aggregation-prone proteins containing expanded polyQ repeats. These proteins can cause cellular toxicity and neurodegeneration through both gain- and loss-of-function mechanisms. Recent evidence indicates widespread loss-of-function of a large number of cellular proteins owing to their aberrant recruitment into the aggregates of polyQ-expanded proteins. Moreover, emerging evidence indicates that polyQ proteins can form coiled coil (CC) domains with critical roles in driving both their aggregation and the interactions with other cellular proteins. Indeed, the interactomes of disease-related polyQ proteins are enriched in CC domains, and some of these have been identified that mediate interactions with polyQ-expanded proteins. These findings suggested to us the possibility that cellular processes relying extensively on CC molecular machineries may be particularly vulnerable to the presence of polyQ-expanded proteins. To identify these machineries and the possible role of CCs in their aberrant recruitment into polyQ aggregates, we have undertaken here a systematic functional classification of the CC proteins of the entire human proteome. This analysis revealed a significant enrichment of CC domains in discrete protein groups associated with specific functional annotations (i.e. 'biological process' (BP) Gene Ontology (GO) terms). In particular, we found that a vast array of cellular processes related to the assembly of structural proteins (e.g. cytoskeletal dynamics, cell division, cilia assembly) and to membrane trafficking (e.g. exo-/endo-cytosis, ER-Golgi trafficking, autophagy, mitochondrial fission/fusion) rely on a high number of functionally interacting CC proteins. Importantly, most of these CC-dependent processes are known to be altered in polyQ diseases. To gain a mechanistic understanding of the role of CC domains in the polyQ-related dysfunction of these compartments, we focused on subsets of CC proteins involved in autophagy and mitochondrial fission/fusion. CC structural predictions, circular dichroism spectropolarimetry, chemical crosslinking, and cellular co-expression analyses, that we performed on key proteins of these two functional groups, identified a number of candidate CC domains that may mediate interactions with polyQ-expanded proteins. We screened, by means of cellular co-expression studies, CC fragments of autophagic and mitochondrial proteins that may be sufficient for the recruitment into polyQ-expanded protein aggregates, and found positive cases of CC-mediated recruitment. Overall, the results of our analyses have a twofold relevance. First, they define a functional atlas of CC proteins of the entire human proteome that identifies known and candidate cellular processes that may be critically affected by polyQ-expanded proteins via aberrant CC interactions. Second, they identify, and structurally characterize defined CC domains with critical roles in the recruitment of autophagic and mitochondrial molecular machineries into polyQ-expanded protein aggregates.