SUMMARY Eukaryotic cells contain large RNA-protein assemblies referred to as RNP granules, whose assembly is promoted by both traditional protein interactions and intrinsically disordered protein domains. Using RNP granules as an example, we provide evidence for an assembly mechanism of large cellular structures wherein specific protein-protein or protein-RNA interactions act together with promiscuous interactions of intrinsically disordered regions (IDRs). This synergistic assembly mechanism illuminates RNP granule assembly and explains why many components of RNP granules, and other large dynamic assemblies, contain IDRs linked to specific protein-protein or protein-RNA interaction modules. We suggest assemblies based on combinations of specific interactions and promiscuous IDRs are common features of eukaryotic cells.

SummaryA dominant histopathological feature in neuromuscular diseases including amyotrophic lateral sclerosis and inclusion body myopathy is cytoplasmic aggregation of the RNA-binding protein TDP-43. Although rare protein-misfolding mutations in TDP-43 often cause protein aggregation, most patients do not have a TDP-43 mutation suggesting aggregates of wild-type TDP-43 arise by an unknown mechanism. Here we show TDP-43 is an essential protein for normal skeletal muscle formation that unexpectedly forms cytoplasmic, amyloid-like oligomeric assemblies, termed myo-granules, during skeletal muscle regeneration in mice and humans. Myo-granules bind mRNAs encoding sarcomeric proteins and are cleared as myofibers mature. Although myo-granules occur during normal skeletal muscle regeneration, myo-granules can seed TDP-43 amyloid fibrils in vitro, and are increased in a mouse model of inclusion body myopathy. Therefore, heightened assembly or decreased clearance of functionally normal myo-granules could be the source of cytoplasmic TDP-43 aggregates common to neuromuscular disease.

RNAs besides tRNA and rRNA contain chemical modifications, including the recently described 5′ nicotinamide-adenine dinucleotide (NAD + ) RNA in bacteria. Whether 5′ NAD-RNA exists in eukaryotes remains unknown. We demonstrate that 5′ NAD-RNA is found on subsets of nuclear and mitochondrial encoded mRNAs in Saccharomyces cerevisiae. NAD-mRNA appears to be produced cotranscriptionally because NAD-RNA is also found on pre-mRNAs, and only on mitochondrial transcripts that are not 5′ end processed. These results define an additional 5′ RNA cap structure in eukaryotes and raise the possibility that this 5′ NAD + cap could modulate RNA stability and translation on specific subclasses of mRNAs.T he number and prevalence of known chemical modifications on mRNAs have dramatically increased in the past several years (1). Quantification of these modification events suggests they occur in many RNAs (2, 3). Importantly, several of these modifications have functional consequences (4-6). For example, the presence of a single N 6 -methyladenosine within the 5′ UTR of an mRNA increases translation initiation (4). In addition, the methylation status of cytosine residues within the 3′ UTR of the p16(INK4) human mRNA affects mRNA stability (6). Due to the increasing sensitivity of RNA sequencing (RNA-Seq) and small-molecule mass spectrometry, it is reasonable to hypothesize that many novel chemical modifications within mRNAs remain to be discovered.One modification recently identified in bacteria is 5′ nicotinamide-adenine dinucleotide (NAD + )-linked RNA (7, 8). Because canonical bacterial RNAs contain a 5′ triphosphate terminus, addition of NAD + to the 5′ end of RNAs represents a rudimentary "capping" mechanism, perhaps designed to impart specific properties for these RNAs by granting them a more structurally complex 5′ end. Consistent with this idea, NAD + addition to RNAs appears to occur during transcription initiation (9), as opposed to the more complex eukaryote 5′ capping, which occurs after transcription has commenced (10). Because the NAD + modification defines the 5′ end of NAD-RNAs, this modification can affect RNA stability in Escherichia coli (7, 9).For decades, 5′ end classification and study of eukaryotic mRNAs have been restricted to canonical 7-methylguanosine (m 7 G) "caps" and their methylated variants (11). The m 7 G cap modulates numerous facets of mRNA metabolism, including stability (12, 13), translation (14, 15), and export (16). The importance of this modification is underscored by the substantial cellular machinery dedicated to its addition and removal (17,18). Thus, mRNAs containing noncanonical 5′ termini may have distinct properties and be subject to alternative metabolic events.Described herein is the identification of NAD-RNAs in the eukaryote Saccharomyces cerevisiae. Examples of NAD-RNAs in S. cerevisiae include nuclear encoded mRNAs for ribosomal proteins, as well as some mitochondrial encoded transcripts. Our data suggest that the NAD + moiety is added during initiation in both nuclear and mitoc...