R Ribonuclease L (RNase L) (OAS) [(Baglioni et al. 1978), reviewed by (Hovanessian and Justesen 2007)]. Melissa Drappier and Thomas Michiels Based on these observations, an RNase de Duve Institute, Université catholique de L activation model was proposed (Fig. 2). In this Louvain, Brussels, Belgium model, virus infection induces IFN expression, which, in turn, triggers the upregulation of OAS expression. dsRNA synthesized in the course of Synonyms viral infection binds OAS, leading to the activa- tion of OAS catalytic activity and to the synthesis PRCA1; RNS4 of 2-5A molecules. 2-5A in turn bind to RNase L, which is present in cells as a latent enzyme. Upon 2-5A binding, RNase L becomes activated by dimerization and cleaves viral and cellular RNA Historical Background (Fig. 2). Since then, cloning of the RNase L gene in In early stages of viral infection, the innate 1993 (Zhou et al. 1993), generation of RNase immune response and particularly the interferon À À L-deficient (RNase L / ) mice in 1997 (Zhou response play a critical role in restricting viral et al. 1997), and solving RNase L structure (Han replication and propagation, awaiting the estab- et al. 2014; Huang et al. 2014) were the major lishment of the adaptive immune response. One of milestones in the understanding of RNase the best-described IFN-dependent antiviral L function. responses is the OAS/RNase L pathway. This two-component system is controlled by type I and type III interferons (IFN). Back in the General Features, Biochemistry, 1970s, the groups of I. Kerr and P. Lengyel dis- and Regulation of RNase L Activity covered a cellular endoribonuclease (RNase) activity that was increased by IFN and depended RNase L is the effector enzyme of the on the presence of double-stranded RNA IFN-induced, OAS/RNase L pathway. The gene (dsRNA) (Brown et al. 1976; Kerr et al. 1977). encoding RNase L is present in higher vertebrates Further, a correlation was found between this 0 0 (birds, reptiles, and mammals) but not in insects or RNase activity and the synthesis of unusual 2 -5 fish. RNase L is constitutively expressed at low oligoadenylates (2-5A) (Fig. 1) by a family of levels in most cell types. Its activation however enzymes called oligoadenylate synthetases depends on 2-5A synthesized by OAS, whose # Springer Science+Business Media LLC 2016 S. Choi (ed.), Encyclopedia of Signaling Molecules, DOI 10.1007/978-1-4614-6438-9_101861-1 2 Ribonuclease L (RNase L) Ribonuclease L (RNase L), Fig. 1 Structure of 20-50 3 adenylyl residues (n 1). Naturally occurring 2-5A oligoadenylates (2-5A). 2-5A are short oligoadenylates molecules contain three phosphate groups at their 50 end, linked by 20,50-phosphodiester bonds. They contain one but a single phosphate is sufficient for RNase L activation to three phosphate groups at their 50 end and at least VIRUS RNA dsRNA 2-5A 2-5A ADP ADP ADP 2-5A dimerization 2-5A 2-5A 9 8 7654 3 2 1 RNA IFN 2-5A 2-5A 9 degradation 8 2 4 9 2-5A 7 1 3 5 6 7 8 6 4 5 2 3 OAS 1 RNase L Ribonuclease L (RNase L), Fig. 2 RNase L activation (i.e., after viral infection). OAS then synthetize 2-5A, pathway. In response to IFN-a/b or IFN-l, OAS gene which bind to ankyrin repeats of RNase L and trigger transcription is upregulated. OAS are produced as inactive RNase L dimerization and activation. Active RNase enzymes but become activated in the presence of dsRNA L degrades viral and cellular RNA basal expression is low but IFN inducible in most [(Le Roy et al. 2007), reviewed by (Ezelle cell types except for dendritic cells and macro- et al. 2016)]. phages where basal OAS levels are higher. RNase RNase L is a 741-amino-acid-long endo- L is typically described as a cytosolic enzyme, but ribonuclease (83,5 kDa) formed of three con- it was also detected in other subcellular compart- served domains (Fig. 3): ments such as the nucleus and mitochondria Ribonuclease L (RNase L) 3 Ribonuclease L (RNase L), Fig. 3 RNase L activation to the pseudokinase domain contributes to RNase and control. RNase L is composed of three domains: the L dimerization. Tight control of RNase L activation N-terminal ankyrin domain; the central pseudokinase includes gene transcription dependence on IFN, OAS domain, which binds nucleotides; and the C-terminal endo- activity dependence on dsRNA, rapid degradation of 2- ribonuclease domain. 2-5A bind ankyrin repeats R2 and 5A by 20 phosphodiesterases (PDE), and inhibition of R4 of one RNase L protomer and R9 and the pseudokinase RNase L by the cellular protein RLI N-terminal lobe of the other protomer. ATP/ADP binding (1) The N-terminal ankyrin domain is composed unfolded protein response (Lee et al. 2008). of nine ankyrin repeats (R1–R9) and acts as Despite this similarity, the pseudokinase the sensor and regulatory domain of the domain is catalytically inactive due to the enzyme. It binds 2-5A, which trigger RNase lack of several residues critical for catalytic L dimerization and activation. Ankyrin kinase activity (Dong and Silverman 1999). repeats are 33-residue motifs consisting of This central domain however binds ADP or two a-helices separated by loops, which usu- ATP and plays a role in RNase L dimerization. ally mediate protein-protein interactions. In (3) The C-terminal domain is the endo- absence of 2-5A, the ANK domain maintains ribonuclease enzymatic domain, responsible RNase L in a monomeric inactive state. for target RNA cleavage. It cleaves cellular (2) The central protein kinase-like or pseudo- and viral RNA with little specificity. It pre- kinase domain exhibits significant similarity dominantly cleaves after UU or UA dinucle- to protein kinases, markedly to Ire1, an ER otides and, more broadly, recognizes the transmembrane sensor implicated in the UN^N pattern, promoting cleavage 30 of UN 4 Ribonuclease L (RNase L) sequence, leaving a 50-OH end and 30-20 Cellular Functions of RNase L cyclic phosphoryl group. (4) Recent structural studies show that 2-5A bind Although RNase L knockout mice are viable and R2 and R4 of one RNase L protomer and R9 display no overt phenotype, it has become clear and the pseudokinase N-terminal lobe of the that RNase L activity is required for fine-tuning of other protomer (Han et al. 2012, 2014; Huang important cellular processes, including cell prolif- et al. 2014). Thus, 2-5A promotes RNase eration and differentiation, apoptosis, or mainte- L self-association by acting as a template nance of cytoskeleton integrity. In human, that fills the interface between two copies of associations have been found between RNase the ANK domain (Fig. 3). L activity and susceptibility to cancer or chronic fatigue syndrome (Fig. 4) [see reviews by (Bisbal and Silverman 2007; Ezelle et al. 2016; Gusho Though some control of its expression exists at et al. 2016)]. the posttranscriptional level, RNase L is constitu- tively expressed as a latent enzyme. Its activity (1) Regulation of mRNA translation and stabil- needs to be tightly regulated since uncontrolled ity: The major effects of RNase L on cellular RNase L activity would lead to cellular RNA processes depend on its nuclease activity, degradation and apoptosis. Several layers of whereby RNase L regulates mRNA turnover. RNase L activity control exist: Recent transcriptomic studies suggest that RNase L exhibits some specificity toward (1) Activation of the pathway can be mediated by some mRNA targets. It was proposed that type I (IFN-a/b) and type III (IFN-l) inter- proteins interacting with RNase L might ferons (IFN), which are generally produced direct the nuclease toward specific cell com- upon viral infection of a tissue. IFNs trigger partments or toward a subset of mRNA. For the upregulation of OAS gene transcription. instance, RNase L regulates mRNA transla- (2) OAS catalytic activity itself depends on the tion through its association with the transla- presence of cytoplasmic dsRNA, which is tion termination factor eRF3 (Le Roy typically produced when a virus replicates in et al. 2005). It can increase both translation the cell. From ATP, activated OAS catalyze read-through at premature termination codons the synthesis of 2-5A, which are very potent and +1 frameshift efficiency, the latter mech- RNase L activators effective at subnanomolar anism being, for instance, involved in the concentrations. expression of ornithine decarboxylase anti- (3) 2-5A are degraded within minutes of their zyme 1 (OAZ1). eRF3 is thought to address synthesis, by 20-phosphodiesterases (PDE) RNase L to polysomes. RNase L also interacts and phosphatases. The presence of intracellu- with tristetraprolin (TTP), a protein known to lar 2-5A is thus transient and directly mirrors interact with AU-rich sequences found in the OAS activity. 30 noncoding regions of short-lived mRN- (4) RNase L was reported to undergo posttrans- A. TTP may thus guide RNase L to mRNA lational modifications like ubiquitylation and carrying AU-rich sequences (Brennan-Laun hydroxylation, which can affect function and et al. 2014). A mitochondrial translation initi- stability. ation factor called IF2mt was identified in a (5) RNase L activity can also be negatively regu- two-hybrid screen as another RNase lated by the RNase L inhibitor (RLI), a cellu- L interactor. This factor would promote lar factor also known as inhibitor-/ATP- RNase L access to the mitochondria where binding cassette, subfamily E member RNase L was shown to degrade mtRNA 1 (ABCE1) (Martinand et al. 1998). (Le Roy et al. 2007). Ribonuclease L (RNase L) 5 Ribonuclease L (RNase L), Fig.