Size Selective Recognition of siRNA by an RNA Silencing Suppressor

RNA silencing in plants likely exists as a defense mechanism against molecular parasites such as RNA viruses, retrotransposons, and transgenes. As a result, many plant viruses have adapted mechanisms to evade and suppress gene silencing. Tombusviruses express a 19 kDa protein (p19), which has been shown to suppress RNA silencing in vivo and bind silencing-generated and synthetic small interferingRNAs (siRNAs) in vitro. Here we report the 2.5 A° crystal structure of p19 from the Carnation Italian ringspot virus (CIRV) bound to a 21 nt siRNA and demonstrate in biochemical and in vivo assays that CIRV p19 protein acts as molecular caliper to specifically select siRNAs based on the length of the duplex region of the RNA

Engineered proteins with Pumilio/ fem-3 mRNA binding factor scaffold to manipulate RNA metabolism

Pumilio/fem-3 mRNA binding factor (FBF) proteins are characterized by a sequence-specific RNA-binding domain. This unique single-stranded RNA recognition module, whose sequence specificity can be reprogrammed, has been fused with functional modules to engineer protein factors with various functions. Here we summarize the advancement in developing RNA regulatory tools and opportunities for the future

Engineering splicing factors with designed specificities

Alternative splicing is generally regulated by trans-acting factors that specifically bind pre-mRNA to activate or inhibit the splicing reaction. This regulation is critical for normal gene expression, and dysregulation of splicing is closely associated with human diseases. Here we engineer artificial splicing factors by combining sequence-specific RNA-binding domains of human Pumilio1 with functional domains that regulate splicing. We applied these factors to modulate different types of alternative splicing in selected targets, examine the activity of effector domains from natural splicing factors, and modulate splicing of an endogenous gene, Bcl-x, an anti-cancer target. The designer factor targeted to Bcl-x increased the pro-apoptotic Bcl-xS splicing isoform, thus promoting apoptosis and increasing chemosensitivity of cancer cells to common anti-tumor drugs. Our approach permits the creation of artificial factors to target virtually any pre-mRNA, providing a new strategy to study splicing regulation and manipulate disease-associated splicing events

Specific and Modular Binding Code for Cytosine Recognition in Pumilio/FBF (PUF) RNA-binding Domains

Pumilio/fem-3 mRNA-binding factor (PUF) proteins possess a recognition code for bases A, U, and G, allowing designed RNA sequence specificity of their modular Pumilio (PUM) repeats. However, recognition side chains in a PUM repeat for cytosine are unknown. Here we report identification of a cytosine-recognition code by screening random amino acid combinations at conserved RNA recognition positions using a yeast three-hybrid system. This C-recognition code is specific and modular as specificity can be transferred to different positions in the RNA recognition sequence. A crystal structure of a modified PUF domain reveals specific contacts between an arginine side chain and the cytosine base. We applied the C-recognition code to design PUF domains that recognize targets with multiple cytosines and to generate engineered splicing factors that modulate alternative splicing. Finally, we identified a divergent yeast PUF protein, Nop9p, that may recognize natural target RNAs with cytosine. This work deepens our understanding of natural PUF protein target recognition and expands the ability to engineer PUF domains to recognize any RNA sequence

The Drosophila hnRNP F/H Homolog Glorund Uses Two Distinct RNA-Binding Modes to Diversify Target Recognition

The Drosophila hnRNP F/H homolog, Glorund (Glo), regulates nanos mRNA translation by interacting with a structured UA-rich motif in the nanos 30 untranslated region. Glo regulates additional RNAs, however, and mammalian homologs bind G-tract sequences to regulate alternative splicing, suggesting that Glo also recognizes G-tract RNA. To gain insight into how Glo recognizes both structured UA-rich and G-tract RNAs, we used mutational analysis guided by crystal structures of Glo’s RNA-binding domains and identified two discrete RNA-binding surfaces that allow Glo to recognize both RNA motifs. By engineering Glo variants that favor a single RNA-binding mode, we show that a subset of Glo’s functions in vivo is mediated solely by the G-tract binding mode, whereas regulation of nanos requires both recognition modes. Our findings suggest a molecular mechanism for the evolution of dual RNA motif recognition in Glo that may be applied to understanding the functional diversity of other RNA-binding proteins

Combinatorial control of messenger RNAs by Pumilio, Nanos and Brain Tumor Proteins

<p>Eukaryotes possess a vast array of RNA-binding proteins (RBPs) that affect mRNAs in diverse ways to control protein expression. Combinatorial regulation of mRNAs by RBPs is emerging as the rule. No example illustrates this as vividly as the partnership of 3 <i>Drosophila</i> RBPs, Pumilio, Nanos and Brain Tumor, which have overlapping functions in development, stem cell maintenance and differentiation, fertility and neurologic processes. Here we synthesize 30 y of research with new insights into their molecular functions and mechanisms of action. First, we provide an overview of the key properties of each RBP. Next, we present a detailed analysis of their collaborative regulatory mechanism using a classic example of the developmental morphogen, <i>hunchback</i>, which is spatially and temporally regulated by the trio during embryogenesis. New biochemical, structural and functional analyses provide insights into RNA recognition, cooperativity, and regulatory mechanisms. We integrate these data into a model of combinatorial RNA binding and regulation of translation and mRNA decay. We then use this information, transcriptome wide analyses and bioinformatics predictions to assess the global impact of Pumilio, Nanos and Brain Tumor on gene regulation. Together, the results support pervasive, dynamic post-transcriptional control.</p