Positioning of pyrimidine motifs around cassette exons defines their PTB-dependent splicing in Arabidopsis

Alternative splicing (AS) is a complex process that generates transcript variants from a single pre-mRNA and is involved in numerous biological functions. Many RNA-binding proteins are known to regulate AS; however, little is known about the underlying mechanisms, especially outside the mammalian clade. Here, we show that polypyrimidine tract binding proteins (PTBs) from Arabidopsis thaliana regulate AS of cassette exons via pyrimidine (Py)-rich motifs close to the alternative splice sites. Mutational studies on three PTB-dependent cassette exon events revealed that only some of the Py motifs in this region are critical for AS. Moreover, in vitro binding of PTBs did not reflect a motif's impact on AS in vivo. Our mutational studies and bioinformatic investigation of all known PTB-regulated cassette exons from A. thaliana and human suggested that the binding position of PTBs relative to a cassette exon defines whether its inclusion or skipping is induced. Accordingly, exon skipping is associated with a higher frequency of Py stretches within the cassette exon, and in human also upstream of it, whereas exon inclusion is characterized by increased Py motif occurrence downstream of said exon. Enrichment of Py motifs downstream of PTB-activated 5' splice sites is also seen for PTB-dependent intron removal and alternative 5' splice site events from A. thaliana, suggesting this is a common step of exon definition. In conclusion, the position-dependent AS regulatory mechanism by PTB homologs has been conserved during the separate evolution of plants and mammals, while other critical features, in particular intron length, have considerably changed.

The leader RNA of SARS-CoV-2 sequesters polypyrimidine tract binding protein (PTBP1) and influences pre-mRNA splicing in infected cells

The large amount of viral RNA produced during infections has the potential to interact with and effectively sequester cellular RNA binding proteins, thereby influencing aspects of post-transcriptional gene regulation in the infected cell. Here we demonstrate that the abundant 5' leader RNA region of SARS-CoV-2 viral RNAs can interact with the cellular polypyrimidine tract binding protein (PTBP1). Interestingly, the effect of a knockdown of PTBP1 protein on cellular gene expression is also mimicked during SARS-CoV-2 infection, suggesting that this protein may be functionally sequestered by viral RNAs. Consistent with this model, the alternative splicing of mRNAs that is normally controlled by PTBP1 is dysregulated during SARS-CoV-2 infection. Collectively, these data suggest that the SARS-CoV-2 leader RNA sequesters the cellular PTBP1 protein during infection, resulting in significant impacts on the RNA biology of the host cell. These alterations in post-transcriptional gene regulation may play a role in SARS-CoV-2 mediated molecular pathogenesis.

A transient in planta editing assay identifies specific binding of the splicing regulator PTB as a prerequisite for cassette exon inclusion

The dynamic interaction of RNA-binding proteins (RBPs) with their target RNAs contributes to the diversity of ribonucleoprotein (RNP) complexes that are involved in a myriad of biological processes. Identifying the RNP components at high resolution and defining their interactions are key to understanding their regulation and function. Expressing fusions between an RBP of interest and an RNA editing enzyme can result in nucleobase changes in target RNAs, representing a recent addition to experimental approaches for profiling RBP/RNA interactions. Here, we have used the MS2 protein/RNA interaction to test four RNA editing proteins for their suitability to detect target RNAs of RBPs in planta. We have established a transient test system for fast and simple quantification of editing events and identified the hyperactive version of the catalytic domain of an adenosine deaminase (hADARcd) as the most suitable editing enzyme. Examining fusions between homologs of polypyrimidine tract binding proteins (PTBs) from Arabidopsis thaliana and hADARcd allowed determining target RNAs with high sensitivity and specificity. Moreover, almost complete editing of a splicing intermediate provided insight into the order of splicing reactions and PTB dependency of this particular splicing event. Addition of sequences for nuclear localisation of the fusion protein increased the editing efficiency, highlighting this approach's potential to identify RBP targets in a compartment-specific manner. Our studies have established the editing-based analysis of interactions between RBPs and their RNA targets in a fast and straightforward assay, offering a new system to study the intricate composition and functions of plant RNPs in vivo.

Polypyrimidine Tract Binding Protein: A Universal Player in Cancer Development

OBJECTIVES

Polypyrimidine tract binding protein is a 57-Kda protein located in the perinucleolar compartment where it binds RNA and regulates several biological functions through the regulation of RNA splicing. Numerous research articles have been published that address the cellular network and functions of PTB and its isoforms in various disease states.

METHODOLOGY

Through an extensive PubMed search, we attempt to summarize the relevant research into this biomolecule.

RESULTS

Besides its roles in embryonic development, neuronal cell growth, RNA metabolism, apoptosis, and hematopoiesis, PTB can affect cancer growth via several metabolic, proliferative, and structural mechanisms. PTB overexpression has been documented in several cancers where it plays a role as a novel prognostic factor.

CONCLUSION

The diverse carcinogenic effect opens an argument into its potential role in inhibitory targeted therapy.

Solution and crystal structures of a C-terminal fragment of the neuronal isoform of the polypyrimidine tract binding protein (nPTB)

The eukaryotic polypyrimidine tract binding protein (PTB) serves primarily as a regulator of alternative splicing of messenger RNA, but is also co-opted to other roles such as RNA localisation and translation initiation from internal ribosome entry sites. The neuronal paralogue of PTB (nPTB) is 75% identical in amino acid sequence with PTB. Although the two proteins have broadly similar RNA binding specificities and effects on RNA splicing, differential expression of PTB and nPTB can lead to the generation of alternatively spliced mRNAs. RNA binding by PTB and nPTB is mediated by four RNA recognition motifs (RRMs). We present here the crystal and solution structures of the C-terminal domain of nPTB (nPTB34) which contains RRMs 3 and 4. As expected the structures are similar to each other and to the solution structure of the equivalent fragment from PTB (PTB34). The result confirms that, as found for PTB, RRMs 3 and 4 of nPTB interact with one another to form a stable unit that presents the RNA-binding surfaces of the component RRMs on opposite sides that face away from each other. The major differences between PTB34 and nPTB34 arise from amino acid side chain substitutions on the exposed β-sheet surfaces and adjoining loops of each RRM, which are likely to modulate interactions with RNA.

A pathway involving HDAC5, cFLIP and caspases regulates expression of the splicing regulator polypyrimidine tract binding protein in the heart

Polypyrimidine tract binding protein (PTB) regulates pre-mRNA splicing, having special relevance for determining gene expression in the differentiating muscle. We have previously shown that PTB protein abundance is progressively reduced during heart development without reduction of its own transcript. Simultaneous reduction of histone deacetylase (HDAC) expression prompted us to investigate the potential link between these events. HDAC5-deficient mice have reduced cardiac PTB protein abundance, and HDAC inhibition in myocytes causes a reduction in endogenous expression of cellular FLICE-like inhibitory protein (cFLIP) and caspase-dependent cleavage of PTB. In agreement with this, cardiac PTB expression is abnormally high in mice with cardiac-specific executioner caspase deficiency, and cFLIP overexpression prevents PTB cleavage in vitro. Caspase-dependent cleavage triggers further fragmentation of PTB, and these fragments accumulate in the presence of proteasome inhibitors. Experimental modification of the above processes in vivo and in vitro results in coherent changes in the alternative splicing of genes encoding tropomyosin-1 (TPM1), tropomyosin-2 (TPM2) and myocyte enhancer factor-2 (MEF2). Thus, we report a pathway connecting HDAC, cFLIP and caspases regulating the progressive disappearance of PTB, which enables the expression of the adult variants of proteins involved in the regulation of contraction and transcription during cardiac muscle development.