**5. Members of the FET family: FUS, EWS, TAF15**

Members of the FET family are very similar RNA-binding proteins containing the following main domains in their structure: a SYGQ-rich N-terminal prion-like domain enriched with uncharged polar amino acids (asparagine, glutamine, serine, and tyrosine) and glycine [36], an RNA-binding motif (RNA-recognition motif, RRM), a "zinc finger" motif, and several RGG domains rich in glycine [37–39]. Three proteins (FUS, EWS, and TAF15) also have a nuclear location signal (NLS) that is recognized by the nuclear receptor transportin and is responsible for protein transport from the cytoplasm into the nucleus and back [27, 40–43]. Genes of the FET family are expressed practically everywhere. Proteins of this family are involved in regulation of different stages of gene expression, including transcription, pre-mRNA splicing, mRNA transport, and also take part in DNA repair [44–46]. The family name stems from three capital letters of its three members: proteins FUS/TLS (fused in sarcoma or translocated in liposarcoma), EWS (Ewing's sarcoma), and TAF15 (TATA-binding protein-associated factor 2N) [47]. These RNA-binding proteins participate in many cell processes, including transcrip‐ tion, pre-mRNA splicing, DNA repair, and mRNA transport in neurons [44–46]. As a rule, under stress conditions such RNA-binding proteins shift from the nucleus into the cytoplasm and take part in the formation of SGs, after which they return to the nucleus. This cycle is multiply repeated during the cell lifetime, and it is not excluded that under such conditions errors may occur in its functioning. The mechanism of activity of FET proteins, their cellular localization, and determination of domains involved in different processes are to be clarified.

Prion-like domains of these proteins associated with human neurodegenerative diseases are critical for their aggregation [32]. Proteins of this family are involved not only in neurodege‐ nerative diseases, such as ALS [33], Huntington's disease, spinocerebellar ataxia, and denta‐ torubral cerebellar atrophy [48], but also in the development of human mixoid liposarcoma [49–51].

#### **5.1. FUS: Its functions, structural peculiarities of a prion-like domain, mutations associated with diseases**

A member of the FET family is the RNA-binding protein FUS, for which a number of mutations associated with ALS have been revealed [27, 40–43, 52]. Under normal conditions, this protein is localized mostly in the nucleus, whereas under pathologic conditions its aggregations are accumulated in the cytoplasm. It was demonstrated on several cell models that FUS delocalized in the cytoplasm was accumulated in SGs. This permitted a conclusion that a high concentra‐ tion of the protein facilitates generation of protein aggregations under FUS pathology [53, 54]. Nonetheless an exact mechanism by which this transformation occurs remains unclear. It is not excluded that prion-like domains affect aggregation-like prions or fibril formation analogous to amyloidogenesis. It is also possible that FUS, accumulated in large amounts, aggregates in the cytoplasm independently of its capacity to sequestration in SGs. The protein accumulation can also facilitate dysfunction of the protein intracellular degradation systems that frequently occurs parallel to neurodegenerative disorders [55]. It should be noted that under sporadic ALS diseases, the control of splicing of a large amount of genes is violated [56]. The FUS protein also regulates alternative splicing, as a rule binding to transcripts containing large introns. Therefore, accumulation of FUS in the cytoplasm can result in the loss of its function in the nucleus and consequently violation of the alternative splicing of a number of genes, e.g., coding proteins associated with the growth and development of axons and the cell cytoskeleton [57]. This version is in agreement with the results obtained on a drosophila model, when an increased FUS concentration in the cell was extremely toxic for organisms that can also cause dysfunction during splicing. In addition, enhanced expression of the gaz gene of the fus ortholog caused death at the pupal stage [58].

In transgenic *Caenorhabditis elegans* worms, in which serine 57 was removed in protein FUS, paralysis occurred much earlier when compared to transgenic species with a full-sized FUS protein [59]. Therewith, the sequence of the FUS protein itself corresponded to that of the FUS protein from the human proteome. This mutation (removal of serine 57) is associated with the sporadic form of ALS [60]. It has been shown that for aggregation and toxicity of yeasts a prionlike (1–239) and an RGG2 (374–422) domains are required, though FUS (1–359) is sufficient for aggregation in models of neuroblastoma cells [54]. Domain RGG3 does not affect aggregation, and mutations in (502–526) lead only to the protein accumulation in the cytoplasm and its insertion in SGs. In yeasts, FUS was accumulated only in the cytoplasm [61]. Some components of SGs, such as translation initiation factors and poly-A-binding proteins, suppress toxicity of FUS aggregations. It is important that many proteins associated with RNA metabolism can affect the toxicity of FUS aggregations.

To verify the hypothesis that dislocation of FUS from the nucleus into the cytoplasm leads to the loss of its function in the nucleus, Murakami et al. created transgenic animals with double gene expression. One of these genes (fus) having the N-terminus, labeled with red fluorescent protein RFP, could not move into the cytoplasm under the action of stress, and the other gene (mutant GFP-fus-P525L) was uniformly distributed both over the nucleus and the cytoplasm. After heat shock, most of GFP-FUS-P525L moved into the SGs, whereas GFP-RUS remained in the cytoplasm. Therewith, the damaging effect was the same as in the experiment with expression of only GFP-fus-P525L. Therefore, the authors concluded that accumulation and aggregation of FUS in the cytoplasm are more neurotoxic than that when FUS lost its function in the nucleus [62].

In any case, formation of such aggregations is the reason for apoptosis, i.e., the process of programmed cell death. Apoptosis is characterized by retaining fragmentation of intracellular components with retention of the integrity of the plasmatic membrane that facilitates fast phagocytosis.

#### **5.2. Role of FET proteins in the formation of stress granules**

under stress conditions such RNA-binding proteins shift from the nucleus into the cytoplasm and take part in the formation of SGs, after which they return to the nucleus. This cycle is multiply repeated during the cell lifetime, and it is not excluded that under such conditions errors may occur in its functioning. The mechanism of activity of FET proteins, their cellular localization, and determination of domains involved in different processes are to be clarified.

Prion-like domains of these proteins associated with human neurodegenerative diseases are critical for their aggregation [32]. Proteins of this family are involved not only in neurodege‐ nerative diseases, such as ALS [33], Huntington's disease, spinocerebellar ataxia, and denta‐ torubral cerebellar atrophy [48], but also in the development of human mixoid liposarcoma

A member of the FET family is the RNA-binding protein FUS, for which a number of mutations associated with ALS have been revealed [27, 40–43, 52]. Under normal conditions, this protein is localized mostly in the nucleus, whereas under pathologic conditions its aggregations are accumulated in the cytoplasm. It was demonstrated on several cell models that FUS delocalized in the cytoplasm was accumulated in SGs. This permitted a conclusion that a high concentra‐ tion of the protein facilitates generation of protein aggregations under FUS pathology [53, 54]. Nonetheless an exact mechanism by which this transformation occurs remains unclear. It is not excluded that prion-like domains affect aggregation-like prions or fibril formation analogous to amyloidogenesis. It is also possible that FUS, accumulated in large amounts, aggregates in the cytoplasm independently of its capacity to sequestration in SGs. The protein accumulation can also facilitate dysfunction of the protein intracellular degradation systems that frequently occurs parallel to neurodegenerative disorders [55]. It should be noted that under sporadic ALS diseases, the control of splicing of a large amount of genes is violated [56]. The FUS protein also regulates alternative splicing, as a rule binding to transcripts containing large introns. Therefore, accumulation of FUS in the cytoplasm can result in the loss of its function in the nucleus and consequently violation of the alternative splicing of a number of genes, e.g., coding proteins associated with the growth and development of axons and the cell cytoskeleton [57]. This version is in agreement with the results obtained on a drosophila model, when an increased FUS concentration in the cell was extremely toxic for organisms that can also cause dysfunction during splicing. In addition, enhanced expression of the gaz gene of

In transgenic *Caenorhabditis elegans* worms, in which serine 57 was removed in protein FUS, paralysis occurred much earlier when compared to transgenic species with a full-sized FUS protein [59]. Therewith, the sequence of the FUS protein itself corresponded to that of the FUS protein from the human proteome. This mutation (removal of serine 57) is associated with the sporadic form of ALS [60]. It has been shown that for aggregation and toxicity of yeasts a prionlike (1–239) and an RGG2 (374–422) domains are required, though FUS (1–359) is sufficient for aggregation in models of neuroblastoma cells [54]. Domain RGG3 does not affect aggregation,

**5.1. FUS: Its functions, structural peculiarities of a prion-like domain, mutations**

[49–51].

**associated with diseases**

106 Update on Amyotrophic Lateral Sclerosis

the fus ortholog caused death at the pupal stage [58].

To understand the mechanism of assembly and disassembly of SGs, it is necessary to know what regions of the chain of RNA-binding proteins can perform the function of a prime and what the role of unstructured regions of simple complexity is. It is worth noting that in many respects protein FUS is a perfect model for studying processes involved in the formation of protein aggregations by the prion mechanism. To search for and reveal properties of prionlike domains, we have chosen three proteins from the FET/TET family of 29 RNA-binding proteins of the human proteome, the structures of which included prion-like domains [31]. The prediction was made using the algorithm developed by Alberti et al. [63], which is based on the choice of protein regions of 60 amino acid residues, similar in the amino acid content to the prion domains of yeast proteins, such as Sup35, Ure2p, and Rnq1p [64]. As a rule, these regions are rich in hydrophilic amino acid residues such as glutamine, asparagine, and tyrosine. In the range of proteins used in the prediction of prion-like domains, the first and second places belonged to FUS and TAF15 among 29 candidates of RNA-binding proteins, and the third was protein EWS [35].

The experiments, devoted to disclosing the capacity of protein FUS to aggregate, demonstrated that upon deletion of the most part of the predicted prion-like domain the protein lost its capacity to self-assemble; however, the formed aggregations did not reveal toxicity (**Figure 2**) [67]. Some components of SGs, such as translation initiation factors and poly-A-binding proteins, suppress toxicity of FUS aggregation [67]. It is important that proteins associated with RNA metabolism can affect the toxicity of FUS aggregations. Prion-like (1–239) and RGG2 (374–422) domains are also required for aggregation and toxicity of yeasts, although FUS (1– 359) is sufficient for simulations on the neuroblastoma cell culture [68]. Domain RGG3 does not affect aggregation, and mutations at (502–526) result only in accumulation of the protein in the cytoplasm and its insertion in SGs [24, 69]. It should be mentioned that in contrast to mammalian cells, in yeast cells protein FUS is accumulated largely in the cytoplasm [61]. This may be connected with the fact that NLS FUS is not recognized by nuclear receptors of yeasts [65]. In mammalian cells, protein FUS is accumulated in the cytoplasm only when it has mutations distorting the reverse transport into the nucleus [69].

**Figure 2.** Effect of different constructions of FUS on aggregation and toxicity in yeasts and aggregation and localization in SGs in cell culture of the SH-SY5Y neuroblastoma [65, 66].

The N-terminal domain of FUS has 27 different variants of GYG, GYS, SYG, and SYS triplets (that can be designated as [G/S]Y[G/S] repeats) [70]. Four mutants were obtained with a different number of substituted tyrosine residues for serine ones to demonstrate that namely tyrosine residues are responsible for the formation of hydrogel. There were 5, 9, 15 substitu‐ tions and all 27. Neither of the mutants could form hydrogel, however, all of them could equally well bind to it. Mutants with substituted residues 5 and 9 could bind to hydrogel, but the remaining mutants could not [70].

#### **5.3. Disordered regions in proteins of the FET family and search for partners for interactions with these proteins**

For all three proteins, the IsUnstruct program [15] predicts the presence of unstructured domains, as a rule, at the N and C termini of the polypeptide chain. Unstructured proteins often play the role of hubs, i.e., have a capacity to concentrate a large number of partners around them (it is accepted that when there are more than five partners, it is a hub) [71]. To what extent is this role validated? To answer this question, it is necessary to determine the presence of functional sites in the protein considered. The search for the number of partners in the STRING database revealed that it exceeds 5 [72] (see **Figure 3**). Usually upon binding to a partner, natively unfolded proteins can acquire a structure that imparts a certain function to them. In other words, the conformation of unstructured proteins is "dictated" by the interac‐ tion partners. This explains their capacity to perform different functions both in the cell and in the extracellular space. For example, the analyzed RNA-binding proteins have regions with large amounts of glycine in addition to large amounts of asparagine, glutamine, and tyrosine, which also facilitates their unfolded state and performance of various functions in the cell because these proteins are involved in the formation of RNP complexes, control of DNA transcription, pre-mRNA splicing, protein posttranslational modification, and many other vital functions [73]. According to the STRING database (version 10), the number of partners is 44 for TAF15, 132 for EWS (**Figure 3**), and 218 for FUS.

(374–422) domains are also required for aggregation and toxicity of yeasts, although FUS (1– 359) is sufficient for simulations on the neuroblastoma cell culture [68]. Domain RGG3 does not affect aggregation, and mutations at (502–526) result only in accumulation of the protein in the cytoplasm and its insertion in SGs [24, 69]. It should be mentioned that in contrast to mammalian cells, in yeast cells protein FUS is accumulated largely in the cytoplasm [61]. This may be connected with the fact that NLS FUS is not recognized by nuclear receptors of yeasts [65]. In mammalian cells, protein FUS is accumulated in the cytoplasm only when it has

**Figure 2.** Effect of different constructions of FUS on aggregation and toxicity in yeasts and aggregation and localization

The N-terminal domain of FUS has 27 different variants of GYG, GYS, SYG, and SYS triplets (that can be designated as [G/S]Y[G/S] repeats) [70]. Four mutants were obtained with a different number of substituted tyrosine residues for serine ones to demonstrate that namely tyrosine residues are responsible for the formation of hydrogel. There were 5, 9, 15 substitu‐ tions and all 27. Neither of the mutants could form hydrogel, however, all of them could equally well bind to it. Mutants with substituted residues 5 and 9 could bind to hydrogel, but the

**5.3. Disordered regions in proteins of the FET family and search for partners for**

For all three proteins, the IsUnstruct program [15] predicts the presence of unstructured domains, as a rule, at the N and C termini of the polypeptide chain. Unstructured proteins often play the role of hubs, i.e., have a capacity to concentrate a large number of partners around them (it is accepted that when there are more than five partners, it is a hub) [71]. To what extent is this role validated? To answer this question, it is necessary to determine the presence of functional sites in the protein considered. The search for the number of partners in the STRING database revealed that it exceeds 5 [72] (see **Figure 3**). Usually upon binding to a partner, natively unfolded proteins can acquire a structure that imparts a certain function to

mutations distorting the reverse transport into the nucleus [69].

in SGs in cell culture of the SH-SY5Y neuroblastoma [65, 66].

remaining mutants could not [70].

108 Update on Amyotrophic Lateral Sclerosis

**interactions with these proteins**

**Figure 3.** The list of partners for EWS has been obtained from the STRING database using the information from the data bases, data about homologies, possible co-expression, experimental confirmation about interactions, etc. The boundary condition for entry in the list of partners is probability of interactions equal to 0.4. Transparency of edges connecting vertices in the graph designates probability of interaction. The more transparent of edge, the less probabili‐ ty of interaction.

These proteins contain many motifs of simple complexity. As shown, the portion of proteins, included in periodic fragments or homorepeats, is an order of magnitude lower in eukaryotic proteomes than in bacterial ones [74, 75]. The proteins with periodic fragments are extremely nonuniformly distributed both over the kingdoms and over organisms within each kingdom [74, 75]. It is worth underlining that these repeated motifs can be located in the region not predicted as prion-like. It is known that protein Ure2 has regions in the carboxy-terminal domain affecting the capacity of the amino-terminal domain to become prion-like [76]. The available data for FUS allow us to conclude that the presence of both a prion-like domain and a C-terminal region corresponding to the RGG2 region (see **Figure 2**) is important for patho‐ logic aggregation. It is most likely that the RNA-binding domain also contributes to the pathologic aggregation. The RNA-binding motif (RRM) in proteins RUS and TAF15 is highly identical and is retained from species to species. On the contrary, the RNA-binding motif (RRM) in protein EWS differs remarkably from other members of the family and does not preserve 100% identity in different species. For example, for FET proteins, the following amyloidogenic regions, revealed using program FoldAmyloid [77], can be indicated in the RNA-binding domain (RRM): AIYVQ/ADFFK/MIHIYL/VEWFD for EWS; TIFVQ/INLYT/ IDWFD for TAF15; and TIFVQ/INLYT/IDWFD for FUS.

The prediction of unstructured regions in proteins of the FET/TET family using program IsUnstruct allowed us to isolate two unstructured domains at the N and C termini and one structured region, corresponding to the RNA-binding domain. **Figure 4** shows probability profiles for amino acid residues of the FET/TET proteins, which make it possible to determine whether they are structured or unstructured. Motifs in the amino acid sequence are shown by different colors. These proteins are characterized by the presence of a large number of homorepeats when one amino acid is recurrent many times. Generally, the longer are the repeats, the higher is the probability that the aggregated protein containing and it is associated with the development of a disease. For example, protein FUS is characterized by five unstruc‐ tured patterns from the pattern library obtained by us from the Protein Data Bank: GSHM, GGGGSGG, GGGGG, GGSGGGGSGGG, and RGGGGSG. The occurrence of these patterns in the given protein in different organisms (human, monkey, pig, mouse, quicken and fish) can be found in our HRaP database, containing data on the occurrence of unstructured patterns and homorepeats in 122 proteomes [14]. For protein TAF15, the glycine-rich recurrent motif is well isolated at the C-terminus: DRGGGYGG/DRSSGGGYSG/DRGSRGGYGG, which is characteristic of many animals and fish [78]. We isolated 22 repeats, and the Uniprot program finds 21 repeats at the C terminus: GRGGRGG/DRGGYGG (**Figure 4**). As concerns EWS, we can observe 14 repeats as SYSQAPS in the prion-like domain (the N-terminal part) and 6 repeats (DRGRGGPGG) in the C-terminal part (**Figure 4**). It should be noted that 15 imperfect repeated motifs (QPGQGYSQQSS) are positioned in a prion-like region (the N-terminal part) and four repeats (DDRRGGRGGY) in the C-terminal one for FUS (**Figure 4**).

As known, protein regions enriched in glycine residues cannot have a rigid spatial structure; therefore, the main function characterizing this protein region is determined by a number of adjacent amino acid residues. It should be noted that in the mentioned proteins with high toxic aggregation, the glycine repeats adjoin arginine, serine, and tyrosine. Domains rich in glycine

Influence of Repeats in the Protein Chain on its Aggregation Capacity for ALS-Associated Proteins http://dx.doi.org/10.5772/63104 111

These proteins contain many motifs of simple complexity. As shown, the portion of proteins, included in periodic fragments or homorepeats, is an order of magnitude lower in eukaryotic proteomes than in bacterial ones [74, 75]. The proteins with periodic fragments are extremely nonuniformly distributed both over the kingdoms and over organisms within each kingdom [74, 75]. It is worth underlining that these repeated motifs can be located in the region not predicted as prion-like. It is known that protein Ure2 has regions in the carboxy-terminal domain affecting the capacity of the amino-terminal domain to become prion-like [76]. The available data for FUS allow us to conclude that the presence of both a prion-like domain and a C-terminal region corresponding to the RGG2 region (see **Figure 2**) is important for patho‐ logic aggregation. It is most likely that the RNA-binding domain also contributes to the pathologic aggregation. The RNA-binding motif (RRM) in proteins RUS and TAF15 is highly identical and is retained from species to species. On the contrary, the RNA-binding motif (RRM) in protein EWS differs remarkably from other members of the family and does not preserve 100% identity in different species. For example, for FET proteins, the following amyloidogenic regions, revealed using program FoldAmyloid [77], can be indicated in the RNA-binding domain (RRM): AIYVQ/ADFFK/MIHIYL/VEWFD for EWS; TIFVQ/INLYT/

The prediction of unstructured regions in proteins of the FET/TET family using program IsUnstruct allowed us to isolate two unstructured domains at the N and C termini and one structured region, corresponding to the RNA-binding domain. **Figure 4** shows probability profiles for amino acid residues of the FET/TET proteins, which make it possible to determine whether they are structured or unstructured. Motifs in the amino acid sequence are shown by different colors. These proteins are characterized by the presence of a large number of homorepeats when one amino acid is recurrent many times. Generally, the longer are the repeats, the higher is the probability that the aggregated protein containing and it is associated with the development of a disease. For example, protein FUS is characterized by five unstruc‐ tured patterns from the pattern library obtained by us from the Protein Data Bank: GSHM, GGGGSGG, GGGGG, GGSGGGGSGGG, and RGGGGSG. The occurrence of these patterns in the given protein in different organisms (human, monkey, pig, mouse, quicken and fish) can be found in our HRaP database, containing data on the occurrence of unstructured patterns and homorepeats in 122 proteomes [14]. For protein TAF15, the glycine-rich recurrent motif is well isolated at the C-terminus: DRGGGYGG/DRSSGGGYSG/DRGSRGGYGG, which is characteristic of many animals and fish [78]. We isolated 22 repeats, and the Uniprot program finds 21 repeats at the C terminus: GRGGRGG/DRGGYGG (**Figure 4**). As concerns EWS, we can observe 14 repeats as SYSQAPS in the prion-like domain (the N-terminal part) and 6 repeats (DRGRGGPGG) in the C-terminal part (**Figure 4**). It should be noted that 15 imperfect repeated motifs (QPGQGYSQQSS) are positioned in a prion-like region (the N-terminal part)

and four repeats (DDRRGGRGGY) in the C-terminal one for FUS (**Figure 4**).

As known, protein regions enriched in glycine residues cannot have a rigid spatial structure; therefore, the main function characterizing this protein region is determined by a number of adjacent amino acid residues. It should be noted that in the mentioned proteins with high toxic aggregation, the glycine repeats adjoin arginine, serine, and tyrosine. Domains rich in glycine

IDWFD for TAF15; and TIFVQ/INLYT/IDWFD for FUS.

110 Update on Amyotrophic Lateral Sclerosis

**Figure 4.** Probability profiles of amino acid residues in proteins of the FET/TET family (A for FUS, B for TAF15, and C for EWS) according to which it is possible to predict possible formation of the structure or its absence by the IsUnstruct program [15].

and arginine (RGG) are known to be responsible for the interaction of proteins with each other and with RNA. As a rule, these interactions are controlled by methylation of arginine [79], whereas phosphorylation of serine residues affects the direct mutual interaction of prion-like domains [45, 80]. Of interest is the fact that deletion of serine (S57), the mutation associated with sporadic form of ALS [81], induced paralysis in transgenic *C. elegans* species, that took place much faster than in transgenic species with a full-sized human FUS [82]. These data make it possible to suggest that phosphorylation and dephosphorylation of serine residues are critical not only for self-assembly of prion domains but also for disassembly by aggregation when required, e.g., termination of stress action on the cell. It was assumed that the presence of tyrosine residues facilitates the formation of hydrogel. In this connection, it should be mentioned that in spite of high similarity of these proteins, relative to other members of the family, the cytoplasmic aggregation of FUS is more toxic that correlates with longer glycine repeats in the amino acid sequence of FUS [33].

#### **5.4. Zinc-finger motif in proteins of the FET family**

All proteins of the FET/TET family are characterized by the presence of a zinc-finger motif. The exclusion is the EWS protein in chickens, because here the zinc-finger motif has not been determined [78]. As known, a classic zinc-finger motif forms a loop, where two cysteine residues and two histidine residues bind zinc ions. The main function of a classic zinc-finger motif is the binding of DNA, which corresponds to its structure consisting of two to three betasheets in the N-terminal region of the protein and one alpha-helix in its C-terminal region. As for the FET/TET family of proteins, the amino acid sequence of the zinc-finger motif in them differs significantly from the classic consensus motif (Cys-X2–4-Cys-X3-Phe-X5-Leu-X2-His-X3- His) [83]. It should be noticed here that the amino acid sequence of this motif in proteins FUS and EWS is highly similar, which is preserved in all organisms studied by us. Our plots on the prediction of the structure demonstrated quite well the correspondence of this motif and the predicted structure (**Figure 4**: proteins FUS and EWS, second peak from the bottom, blue). On the contrary, in TAF15, this motif differs somewhat not only from FUS and EWS but also in the organisms studied by us; according to our prediction, it forms no structure (**Figure 4**). It is important that in proteins of the FET/TET family the zinc-finger motif occurs only once, contrary to the classic variant when it occurs as tandem repeats. As a rule, if the zinc-finger motif occurs once and its sequence differs considerably from the canonical one, the functions of this motif can differ remarkably from the classic motif. For example, it can both be bound to RNA and have no relation to the binding of nucleic acids [83]. The removal of this motif together with the terminal part of the FUS molecule did not affect the protein ability to aggregate and have toxicity either in yeasts or in the cell culture of the SH-SY5Y neuroblastoma [67]. Additional studies should be conducted to reveal the functions of the zinc-finger motif in proteins of the FET/TET family.
