**4. The role of RNA-binding proteins with prion-like domains in diseases**

at regular intervals. A striking example of waiting till stress is over in the formation of stress granules (SG), when the nontranslated mRNA and RNA-binding proteins are assembled in ribonucleoprotein (RNP) complexes in order to terminate protein synthesis and thereby maintain cell energy. In this case, only those proteins that are synthesized are required for cell survival [23]. This means that after termination of the stress action, SGs disintegrate rapidly and the "released" mRNA resumes its functioning. However, if due to some reasons, the residence time of such proteins in SGs increases or their concentration in SGs exceeds the norm, disintegration of SGs can be impeded, creating favorable conditions for generation of the "center of aggregation initiation" that may induce transition to irreversible pathologic protein aggregation [24]. A detailed analysis of the mechanism of assembly and disassembly of SGs can provide a new insight into the development of diseases associated with this process and suggest novel therapeutic approaches. Since the main function of SGs is protection of cells from stress, many investigations are conducted to reveal factors shifting the balance of

104 Update on Amyotrophic Lateral Sclerosis

reversible aggregation towards pathology after stress termination (**Figure 1**).

**Figure 1.** Schematic representation of different conformational states of self-assembly and disintegration of prion-like domains and a possible transition to irreversible pathologic aggregation. Modified and adapted from Li et al. [25].

It is assumed that due to self-assembly, RNA-binding proteins can facilitate formation of SGs using prion-like domains [26–28]. At any rate, it has been established that namely the struc‐ tured part of RNA-binding proteins is responsible for the formation of hydrogel and binding to it [29]. Thus, protein FUS maintained its capacity to form gel even without the removal of the C-terminal region that corresponds to the RNA-binding domain and lost this capacity upon removal of the N-terminal unstructured region corresponding to the prion-like domain. The capacity to bind to hydrogel has been established for proteins such as hnRNPA2, RBM3 RNAbinding proteins, hnRNPA1, TIA1, CPEB2, FMRP, CIRBP, TDP43, and yeast Sup35. The formation of hydrogel was also demonstrated for the hnRNPA2 protein, and it was shown that proteins are retained to a different degree by hydrogel formed of different proteins [29].

Studies of the human genome have allowed isolating a set of RNA-binding proteins with a canonical RNA-binding motif. For example, prion-like domains were predicted for 29 of the 210 RNA-binding proteins for human diseases [30]. Ten of these 29 proteins are associated with neurodegenerative diseases, i.e., proteinopathies [31]. In this chapter, we focus our attention on isolation of repeats in well-studied proteins of the FET/TET family that are included in this group of 10. To elucidate the mechanism of pathologic aggregation of proteins, we have set ourselves the task to determine what repeats of amino acid residues in RNAbinding proteins can be responsible for reversible and irreversible aggregation.

The prion-like domains predicted for TDP-43 and FUS overlap with the region containing a large number of glycine residues. The prion-like domains are critical for aggregation of these proteins associated with human neurodegenerative diseases [32]. It was found that protein TAF15 is also involved in the development of disorder such as amyotrophic lateral sclerosis (ALS) or Lou Gering's disease [33]. At first, this protein was discovered using a bioinformatics analysis as a possible candidate and later it was revealed in ALS patients. It turned out that this protein has very similar properties to those of TDP-43 and FUS proteins involved in this disease. The only difference is that mutations of the protein associated with this disease are capable of higher aggregation in vitro than the wild-type protein, exert a stronger effect on the lifetime, and lead to incorrect localization of proteins in the spinal cord of mammals [33].

Among 29 candidates of RNA-binding proteins with predicted prion-like domains, the first and second places belong to FUS and TAF15, whereas TDP-43 is the 10th in this list [34]. Probably, it is expedient to pay attention to the first 10 proteins because the third place belongs to protein EWS [35]. They should also be detected in ALS patients.
