Roles for Drosophila cap1 2'-O-ribose methyltransferase in the small RNA silencing pathway associated with Argonaute 2

2'-O-methyltransferase-deficient yellow fever virus: Restricted replication in the midgut and secondary tissues of Aedes aegypti mosquitoes severely limits dissemination

The RNA genome of orthoflaviviruses encodes a methyltransferase within the non-structural protein NS5, which is involved in 2'-O-methylation of the 5'-terminal nucleotide of the viral genome resulting in a cap1 structure. While a 2'-O-unmethylated cap0 structure is recognized in vertebrates by the RNA sensor RIG-I, the cap1 structure allows orthoflaviviruses to evade the vertebrate innate immune system. Here, we analyzed whether the cap0 structure is also recognized in mosquitoes. Replication analyses of 2'-O-methyltransferase deficient yellow fever virus mutants (YFV NS5-E218A) of the vaccine 17D and the wild-type Asibi strain in mosquito cells revealed a distinct downregulation of the cap0 viruses. Interestingly, the level of inhibition differed for various mosquito cells. The most striking difference was found in Aedes albopictus-derived C6/36 cells with YFV-17D cap0 replication being completely blocked. Replication of YFV-Asibi cap0 was also suppressed in mosquito cells but to a lower extent. Analyses using chimeras between YFV-17D and YFV-Asibi suggest that a synergistic effect of several mutations across the viral genome accompanied by a faster initial growth rate of YFV-Asibi cap1 correlates with the lower level of YFV-Asibi cap0 attenuation. Viral growth analyses in Dicer-2 knockout cells demonstrated that Dicer-2 is entirely dispensable for attenuating the YFV cap0 viruses. Translation of a replication-incompetent cap0 reporter YFV-17D genome was reduced in mosquito cells, indicating a cap0 sensing translation regulation mechanism. Further, oral infection of Aedes aegypti mosquitoes resulted in lower infection rates for YFV-Asibi cap0. The latter is related to lower viral loads found in the midguts, which largely diminished dissemination to secondary tissues. After intrathoracic infection, YFV-Asibi cap0 replicated slower and to decreased amounts in secondary tissues compared to YFV-Asibi cap1. These results suggest the existence of an ubiquitously expressed innate antiviral protein recognizing 5'-terminal RNA cap-modifications in mosquitoes, both in the midgut as well as in secondary tissues.

Key arginine residues in R2D2 dsRBD1 and dsRBD2 lead the siRNA recognition in Drosophila melanogaster RNAi pathway

In Drosophila melanogaster, Dcr-2:R2D2 heterodimer binds to the 21 nucleotide siRNA duplex to form the R2D2/Dcr-2 Initiator (RDI) complex, which is critical for the initiation of siRNA-induced silencing complex (RISC) assembly. During RDI complex formation, R2D2, a protein that contains three dsRNA binding domains (dsRBD), senses two aspects of the siRNA: thermodynamically more stable end (asymmetry sensing) and the 5'-phosphate (5'-P) recognition. Despite several detailed studies to date, the molecular determinants arising from R2D2 for performing these two tasks remain elusive. In this study, we have performed structural, biophysical, and biochemical characterization of R2D2 dsRBDs. We found that the solution NMR-derived structure of R2D2 dsRBD1 yielded a canonical α1-β1-β2-β3-α2 fold, wherein two arginine salt bridges provide additional stability to the R2D2 dsRBD1. Furthermore, we show that R2D2 dsRBD1 interacts with thermodynamically asymmetric siRNA duplex independent of its 5'-phosphorylation state, whereas R2D2 dsRBD2 prefers to interact with 5'-P siRNA duplex. The mutation of key arginine residues, R53 and R101, in concatenated dsRBDs of R2D2 results in a significant loss of siRNA duplex recognition. Our study deciphers the active roles of R2D2 dsRBDs by showing that dsRBD1 initiates siRNA recognition, whereas dsRBD2 senses 5'-phosphate as an authentic mark on functional siRNA.

The RNA cap methyltransferases RNMT and CMTR1 co-ordinate gene expression during neural differentiation

Regulation of RNA cap formation has potent impacts on gene regulation, controlling which transcripts are expressed, processed and translated into protein. Recently, the RNA cap methyltransferases RNA guanine-7 methyltransferase (RNMT) and cap-specific mRNA (nucleoside-2'-O-)-methyltransferase 1 (CMTR1) have been found to be independently regulated during embryonic stem (ES) cell differentiation controlling the expression of overlapping and distinct protein families. During neural differentiation, RNMT is repressed and CMTR1 is up-regulated. RNMT promotes expression of the pluripotency-associated gene products; repression of the RNMT complex (RNMT-RAM) is required for repression of these RNAs and proteins during differentiation. The predominant RNA targets of CMTR1 encode the histones and ribosomal proteins (RPs). CMTR1 up-regulation is required to maintain the expression of histones and RPs during differentiation and to maintain DNA replication, RNA translation and cell proliferation. Thus the co-ordinate regulation of RNMT and CMTR1 is required for different aspects of ES cell differentiation. In this review, we discuss the mechanisms by which RNMT and CMTR1 are independently regulated during ES cell differentiation and explore how this influences the co-ordinated gene regulation required of emerging cell lineages.

CMTr cap-adjacent 2'-O-ribose mRNA methyltransferases are required for reward learning and mRNA localization to synapses

Cap-adjacent nucleotides of animal, protist and viral mRNAs can be O-methylated at the 2‘ position of the ribose (cOMe). The functions of cOMe in animals, however, remain largely unknown. Here we show that the two cap methyltransferases (CMTr1 and CMTr2) of Drosophila can methylate the ribose of the first nucleotide in mRNA. Double-mutant flies lack cOMe but are viable. Consistent with prominent neuronal expression, they have a reward learning defect that can be rescued by conditional expression in mushroom body neurons before training. Among CMTr targets are cell adhesion and signaling molecules. Many are relevant for learning, and are also targets of Fragile X Mental Retardation Protein (FMRP). Like FMRP, cOMe is required for localization of untranslated mRNAs to synapses and enhances binding of the cap binding complex in the nucleus. Hence, our study reveals a mechanism to co-transcriptionally prime mRNAs by cOMe for localized protein synthesis at synapses.

The two cap methyltransferases (CMTrs) redundantly methylate riboses of first cap adjacent nucleotides in messenger RNAs in Drosophila. Here, CMTrs are required for reward learning and localization of untranslated messenger RNAs to synapses.