Our last review was recently published in Nature Reviews – Molecular and Cell Biology. We discuss about the recurrent identification of unorthodox RBPs by proteome-wide methods to identify proteins bound to RNA, and discuss about the potential biological meaning of this exciting discovery.
What can we expect from the discovery of so many new RBPs? Some might side with Miranda from Shakespeare’s The Tempest and marvel at these novel and goodly RBPs that populate the RNA interactome. Others might think of Huxley’s brave new world and fear dystopia, considering the newly discovered RBPs as nonconformist misfits lacking biological function. Which roles do these new RBPs play?
Hentze MW, Castello A, Schwarzl T, Preiss T.
RBDmap employs UV crosslinking, oligo(dT) selection, partial proteolysis and quantitative proteomics to identify the protein regions engaged in RNA binding in a system-wide scale. Applied to HeLa cells it reported 1,174 RNA-binding sites mapping to 529 RBPs, many of which lacking known RNA-binding domains. A detail RBDmap protocol has now been released for the community in Nature protocols
“The use of RBDmap can now be extended to other cell lines or organisms and can be used to profile in a global scale the behaviour of RNA-binding domains in response to different physiological conditions and stresses.”
Identification of RNA-binding domains of RNA-binding proteins in cultured cells on a system-wide scale with RBDmap. Castello A, Frese CK, Fischer B, Järvelin AI, Horos R, Alleaume AM, Foehr S, Curk T, Krijgsveld J, Hentze MW. Nat Protoc. 2017 Dec;12(12):2447-2464. doi: 10.1038/nprot.2017.106. Epub 2017 Nov 2. PMID: 29095441
The E3 ubiquitin ligase TRIM25 is an antiviral factor recently discovered to bind RNA by the RNA interactome studies (Castello et al., 2012 and Kwon et al., 2013). In a recent work led by our collaborator Gracjan Michlewsky (Wellcome Centre for Cell Biology, University of Edinburgh), we dissected how this protein binds to RNA and what are the consequences of this interaction in TRIM25 function. We discovered that TRIM25 binds RNA via its PRY/SPRY domain and that the interaction with RNA enhances TRIM25 E3 ligase activity, which is necessary for its antiviral role. Using CLIP analyses we showed that TRIM25 binds G-rich sequences present in hundreds of cellular RNAs. Moreover, We discovered that TRIM25 controls the levels of a key component in the interferon response pathway, ZAP (also known as PARP13 and ZC3HAV1).
In conclusion, the E3 ligase activity of TRIM25 is controlled by RNA, breaking once more the view that proteins act on RNA and not the opposite.
RNA-binding activity of TRIM25 is mediated by its PRY/SPRY domain and is required for ubiquitination
Nila Roy Choudhury, Gregory Heikel, Maryia Trubitsyna, Peter Kubik, Jakub S. Nowak, Shaun Webb, Sander Granneman, Christos Spanos, Juri Rappsilber, Alfredo Castello and Gracjan Michlewski
In a recent work with Yolanda Revilla’s lab (CBMSO, Madrid) published in the Journal of Virology, we investigated the role in RNA metabolism of a protein from a complex DNA virus, called African swine fever virus (ASFV). This protein exhibits high homology with cellular decapping enzymes and thus can potentially remove the cap structure from the RNA body triggering degradation. We show that this protein interacts with viral and cellular mRNAs in infected cells. This interaction results in decreased levels of both types of transcripts, agreeing with a putative role as virus-encoded decapping activity. We propose that the degradation of RNA triggered by this protein is key to control gene expression in ASFV infected cells.
It is challenging to determine the composition of a given ribonucleoprotein. We recently approached this problem by adapting the original RNA interactome capture protocol (Castello et al., Cell 2012), to the use of specific antisense LNA probes to capture specific RNA species. We use this method to elucidate the composition of luciferase containing reporters and ribosomal RNA in vitro and in vivo. We were able to recapitulate well-established protein-RNA interactions and to discover new ones.
Specific RNP capture with antisense LNA/DNA mixmers. Rogell B, Fischer B, Rettel M, Krijgsveld J, Castello A, Hentze MW. RNA. 2017 Aug;23(8):1290-1302. doi: 10.1261/rna.060798.117. Epub 2017 May 5.