The target specificity of the RNA binding protein Pumilio is determined by distinct co-factors

Regulation of eukaryotic mRNA deadenylation and degradation by the Ccr4-Not complex

Accurate and precise regulation of gene expression programmes in eukaryotes involves the coordinated control of transcription, mRNA stability and translation. In recent years, significant progress has been made about the role of sequence elements in the 3' untranslated region for the regulation of mRNA degradation, and a model has emerged in which recruitment of the Ccr4-Not complex is the critical step in the regulation of mRNA decay. Recruitment of the Ccr4-Not complex to a target mRNA results in deadenylation mediated by the Caf1 and Ccr4 catalytic subunits of the complex. Following deadenylation, the 5' cap structure is removed, and the mRNA subjected to 5'-3' degradation. Here, the role of the human Ccr4-Not complex in cytoplasmic deadenylation of mRNA is reviewed, with a particular focus on mechanisms of its recruitment to mRNA by sequence motifs in the 3' untranslated region, codon usage, as well as general mechanisms involving the poly(A) tail.

An improved in vivo tethering assay with single molecule FISH reveals that a nematode Nanos enhances reporter expression and mRNA stability

Robust methods are critical for testing the in vivo regulatory mechanism of RNA binding proteins. Here we report improvement of a protein-mRNA tethering assay to probe the function of an RNA binding protein in its natural context within the C. elegans adult germline. The assay relies on a dual reporter expressing two mRNAs from a single promoter and resolved by trans-splicing. The gfp reporter 3'UTR harbors functional binding elements for λN22 peptide, while the mCherry reporter 3'UTR carries mutated nonfunctional elements. This strategy enables internally controlled quantitation of reporter protein by immunofluorescence and mRNA by smFISH. To test the new system, we analyzed a C. elegans Nanos protein, NOS-3, which serves as a post-transcriptional regulator of germ cell fate. Unexpectedly, tethered NOS-3 enhanced reporter expression. We confirmed this enhancement activity with a second reporter engineered at an endogenous germline gene. NOS-3 enhancement of reporter expression was associated with its amino-terminal intrinsically disordered region, not its carboxy-terminal zinc fingers. RNA quantitation revealed that tethered NOS-3 enhances stability of the reporter mRNA. We suggest that this direct NOS-3 enhancement activity may explain a paradox: Classically Nanos proteins are expected to repress RNA, but nos-3 had been found to promote gld-1 expression, an effect that could be direct. Regardless, the new dual reporter dramatically improves in situ quantitation of reporter expression after RNA binding protein tethering to determine its molecular mechanism in a multicellular tissue.

Negative regulation of diminutive cancer regulator through differentiation and microRNA pathway components in Drosophila cells

Drosophila model is intensively studied for the development of cancer. The diminutive (dMyc), a homolog of the human MYC gene, is responsible for cell- apoptosis and its upregulation is responsible for determining the fate of cancerous growth in humans and Drosophila model. This work implores the requirement of dMyc and its expression as one of the major regulator of cancer with other proteins and repression of dMyc mRNA in Drosophila S2 cells. Here we report protein complex of Argonaute 1 (AGO1), Bag of marbles (Bam), and Brain tumor (Brat) proteins and not the individual factor of this complex repression of dMyc mRNA in Drosophila Schneider 2 cells and promote differentiation in cystoblast of Drosophila ovary. These results exhibit the significant role of this complex, including master differentiation factor Bam with other various differentiation factor Brat and microRNA pathway component AGO1, which may negatively regulate dMyc mRNA and so the dMyc protein.

Mechanisms ensuring robust repression of the Drosophila female germline stem cell maintenance factor Nanos via posttranscriptional regulation

During oogenesis in the Drosophila ovary, numerous translational regulators promote the self-renewal or differentiation of stem cells. An intriguing question is how these regulators combine to execute translational regulation. Here, we study mechanisms for the posttranscriptional regulation of nos, a critical stem cell self-renewal factor in the Drosophila ovary; specifically, regulators that promote differentiation of the stem cell daughter. Previous studies showed that Bam, Bgcn, Mei-P26, and Sxl form a complex and repress nos expression through the nos 3'UTR. To further elucidate mechanistic processes of Nos translational regulation, we reconstituted nos repression in cultured Drosophila cells. We identify Ago1 and Brat as new members, and show that Ago1 acts through miRNA binding sites in the proximal region of the nos 3'UTR, whereas Sxl acts via an Sxl binding sequence in the distal region. Combining these findings with published reports, we propose that additional factors Bam, Bgcn, Mei-P26, and Brat are recruited to nos mRNAs through interaction with Ago1 and Sxl. These findings elucidate mechanisms of nos regulation by diverse translational repressors.

Diverse Roles of PUF Proteins in Germline Stem and Progenitor Cell Development in C. elegans

Stem cell development depends on post-transcriptional regulation mediated by RNA-binding proteins (RBPs) (Zhang et al., 1997; Forbes and Lehmann, 1998; Okano et al., 2005; Ratti et al., 2006; Kwon et al., 2013). Pumilio and FBF (PUF) family RBPs are highly conserved post-transcriptional regulators that are critical for stem cell maintenance (Wickens et al., 2002; Quenault et al., 2011). The RNA-binding domains of PUF proteins recognize a family of related sequence motifs in the target mRNAs, yet individual PUF proteins have clearly distinct biological functions (Lu et al., 2009; Wang et al., 2018). The C. elegans germline is a simple and powerful model system for analyzing regulation of stem cell development. Studies in C. elegans uncovered specific physiological roles for PUFs expressed in the germline stem cells ranging from control of proliferation and differentiation to regulation of the sperm/oocyte decision. Importantly, recent studies started to illuminate the mechanisms behind PUF functional divergence. This review summarizes the many roles of PUF-8, FBF-1, and FBF-2 in germline stem and progenitor cells (SPCs) and discusses the factors accounting for their distinct biological functions. PUF proteins are conserved in evolution, and insights into PUF-mediated regulation provided by the C. elegans model system are likely relevant for other organisms.

Functions, mechanisms and regulation of Pumilio/Puf family RNA binding proteins: a comprehensive review

The Pumilio (Pum)/Puf family proteins are ubiquitously present across eukaryotes, including yeast, plants and humans. They generally bind to the 3' untranslated regions of single stranded RNA targets in a sequence specific manner and destabilize them, although a few reports suggest their role in stabilizing the target transcripts. The Pum isoforms are implicated in a wide array of biological processes including stem cell maintenance, development, ribosome biogenesis as well as human diseases. Further studies on Pum would be interesting and important to understand their evolutionarily conserved and divergent features across species, which can have potential implications in medicine, plant sciences as well as basic molecular and cell biological studies. A large number of research reports exists, pertaining to various aspects of Pum, in individual experimental systems. This review is a comprehensive summary of the functions, types, mechanism of action as well as the regulation of Pum in various species. Also, the research questions to be addressed in future are discussed.