veterinary sciences Review Interplay between RNA Viruses and Promyelocytic Leukemia Nuclear Bodies Sabari Nath Neerukonda † Department of Animal and Food and Sciences, University of Delaware, Newark, DE 19716, USA; [email protected] † Current Address: Center for Biologics Evaluation and Research, U.S. Food and Drug Administration, Silver Spring, MD 20993, USA. Abstract: Promyelocytic leukemia nuclear bodies (PML NBs) are nuclear membrane-less sub struc- tures that play a critical role in diverse cellular pathways including cell proliferation, DNA damage, apoptosis, transcriptional regulation, stem cell renewal, alternative lengthening of telomeres, chro- matin organization, epigenetic regulation, protein turnover, autophagy, intrinsic and innate antiviral immunity. While intrinsic and innate immune functions of PML NBs or PML NB core proteins are well defined in the context of nuclear replicating DNA viruses, several studies also confirm their substantial roles in the context of RNA viruses. In the present review, antiviral activities of PML NBs or its core proteins on diverse RNA viruses that replicate in cytoplasm or the nucleus were discussed. In addition, viral counter mechanisms that reorganize PML NBs, and specifically how viruses usurp PML NB functions in order to create a cellular environment favorable for replication and pathogenesis, are also discussed. Keywords: promyelocytic leukemia; nuclear bodies; RNA virus; small ubiquitin modifier; innate immunity; proteasome Citation: Neerukonda, S.N. Interplay between RNA Viruses and Promyelocytic Leukemia Nuclear Bodies. Vet. Sci. 2021, 8, 57. https:// 1. Introduction doi.org/10.3390/vetsci8040057 Promyelocytic leukemia (PML) nuclear bodies (NBs) are observed as 5–30 membrane- less, nuclear matrix-associated, interchromatin speckles with a diameter of 0.1–2 µm in Academic Editor: Daniel Moura a majority of mammalian cell types [1]. Depending upon the cell type, cell cycle phase, de Aguiar and the physiological state, PML NB size, number and localization can vary [2]. The main scaffold protein of PML NBs is the PML protein, which belongs to the TRIM (TRIpartite Received: 18 February 2021 Accepted: 30 March 2021 Motif) family of proteins that are characterized by the presence of a highly conserved RBCC Published: 31 March 2021 motif made of tandemly arranged RING finger domain (R), cysteine-histidine rich B box domains (B1/B2) and an alpha-helical coiled coil domain (CC) (Figure1A) [ 2,3]. Alternative Publisher’s Note: MDPI stays neutral mRNA splicing generates 7 different PML isoforms (I-VII) that differ in C-terminus but with regard to jurisdictional claims in contain identical RBCC motifs in the N-terminus [4,5]. Isoforms (e.g., PML-VII) that lack published maps and institutional affil- C-terminal nuclear localization signal (NLS) remain cytoplasmic [4]. PML is subject to iations. multiple post-translational modifications including SUMOylation, the covalent attachment of a 11 kDa small ubiquitin-like modifier (SUMO) proteins to a site-specific lysine amino acid (aa) residue. Four SUMO protein isoforms, SUMO-1, -2, -3 and -5, have been identified to be conjugated to PML at different lysines (K) with distinct functional consequences [6]. The main PML SUMOylation sites are K65, K160, and K490, although other sites such Copyright: © 2021 by the author. Licensee MDPI, Basel, Switzerland. as K616 have also been reported [7,8]. PML itself is a SUMO E3 ligase [9]. PML also This article is an open access article comprises a SUMO interacting motif (SIM) (aa. 556–562), which enables it to interact with distributed under the terms and SUMOylated PML and SUMOylated client proteins [10]. Through SUMO-SIM interactions, conditions of the Creative Commons PML forms the main scaffold organizer of PML NBs (Figure1B). Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Vet. Sci. 2021, 8, 57. https://doi.org/10.3390/vetsci8040057 https://www.mdpi.com/journal/vetsci Vet. Sci. 2021, 8, 57 2 of 17 Vet. Sci. 2021, 8, x FOR PEER REVIEW 2 of 18 Figure 1. Structure of PromyelocyticFigure 1. Structure leukemia of (PML) Promyelocytic protein and leukemia PML nuclear(PML) protein bodies and (NBs). PML ( Anuclear) All PML bodies isoforms (NBs). (A) All PML isoforms possess a conserved RBCC/TRIM motif in the N-terminus and a variable C-ter- possess a conserved RBCC/TRIM motif in the N-terminus and a variable C-terminus generated due to alternative splicing. minus generated due to alternative splicing. SUMOylation sites (K65, K160, K490, and K616) are SUMOylation sites (K65, K160, K490, and K616) are indicated. Nuclear localization signal (NLS) and SUMO-interacting indicated. Nuclear localization signal (NLS) and SUMO-interacting motifs (SIM) are indicated. motifs (SIM) are indicated.PML binding binding with with client client protein protein via via SUMO-SIM SUMO-SIM interaction interaction and and vice vice versa versa is indicated is indicated by bydouble- double-edged arrows. PMLedged protein arrows. is not PML drawn protein to scale. is not (B drawn) Canonical to scale. PML (B) NBsCanonical are made PML of NBs an are outer made shell of thatan outer is formed by PML oligomers thatshell are that covalently is formed crosslinked by PML byoligomers disulphide that bonds are covalently and display crosslinked non-covalent by disulphide RBCC interactions. bonds and Ubc9 recruitment and SUMOylationdisplay non-covalent of PML results RBCC in interactions. PML NB stabilization Ubc9 recruitment and client and protein SUMOylation recruitment of PML via results client in SIM-SUMOylated PML interactions.PML NB stabilization SUMO1 modifications and client protein are mainly recruitmen presentt via on client the outer SIM-SUMOylated shell, whereas PML SUMO2/3 interactions. modifications in the shell alsoSUMO1 protrude modifications into PML NB are interior. mainly PML present NB ison simplified the outer forshell, understanding whereas SUMO2/3 and does modifications not reflect in several core PML components,the including shell alsoDaxx protrude and Sp100.into PML (C) NB Viral interior. proteins PML are NB known is simplified to colocalize for understanding with PML NBs. and Viral does not reflect several core PML components, including Daxx and Sp100. (C) Viral proteins are known proteins known to interact with PML core components but undetermined for PML NB colocalization were identified with a Vet. Sci. 2021, 8, 57 3 of 17 dashed border. Viral proteins belonging to viruses taxonomically grouped in one family are identified by one color. ZIKV: Zika virus; DENV: Dengue virus; HCV: Hepatitis C virus; EV71: Enterovirus 71; EMCV: Encephalomyocarditis virus; IAV: Influenza A virus; RABV: Rabies virus; LCMV: Lymphocytic choriomeningitis virus; LASV: Lassa virus; RV: Rotavirus; HFV: Human foamy virus; HTLV: Human T-cell lymphotropic virus; ASV: Avian sarcoma virus; PUUV: Puumula virus; HTNV: Hantaan virus; TULV: Tula virus; NS or NSP: Nonstructural protein; C: Core; 3Cpro: 3C protease; 3Dpol: 3D polymerase; M1: matrix; NP or N: Nucleoprotein; P: Phosphoprotein; Z: Zinc finger protein; IN: integrase. PML NB formation involves covalent disulphide linkage of oxidized PML monomers and non-covalent interactions between RBCC domains leading to multimeric oligomeriza- tion to form a PML NB outer shell [11–13]. RBCC oligomerization triggers the recruitment of Ubc9, a SUMO E2 conjugating enzyme that SUMOylates PML, leading to further PML– PML interactions via SUMO-SIM interactions [14]. Furthermore, SUMOylated PML recruits SIM-containing client proteins into an NB inner core via SUMO-SIM interactions, ultimately resulting in liquid–liquid phase separation into mature PML NBs (Figure1B) [ 2]. Mature PML NBs therefore serve as SUMOylation hotspots, but can also promote other mod- ifications such as phosphorylation, acetylation, methylation, ubiquitination, or protein degradation. PML SUMOylation is essential for mature NB formation and functions; PML mutants deficient of SUMOylation (3KR) or SIM interactions (DSIM) still form PML NBs exactly as wild type (WT) PML, although 3KR failed to recruit key PML NB interaction partners Sp100 and Daxx [15,16]. Mature PML NBs function in various cellular pathways, including cell proliferation, DNA damage, apoptosis, transcriptional regulation, stem cell renewal, alternative lengthening of telomeres, chromatin organization, epigenetic regulation, autophagy, intrinsic and innate antiviral immunity [2]. Type I interferons (IFNs) play a critical role in mediating innate immunity against viruses by the induction of several IFN stimulated genes (ISGs) that function at many levels of virus multiplication cycles, including entry, transcription, translation, genome replication, assembly, and release [17]. IFN production occurs upon sensing viral genomes or replication intermediates by pattern recognition receptors to initiate downstream sig- naling and IFN-β production [17,18]. PML-II specifically associates with transcription factors NFκB and STAT1, as well as the CBP, to facilitate transcriptional complex formation on gene promoters of IFN-β and numerous ISGs for an efficient induction of transcrip- tion [19]. PML-IV also potentiates IRF3 induction of IFN-β by sequestering Pin1 to PML NBs, therefore preventing Pin1 negative regulation of IRF3 via ubiquitin dependent degra- dation [20,21]. PML was found to positively regulate not only type I IFN signaling, but also type II IFN (IFN-γ) signaling [22–25]. In fact, PML NB components (PML, Sp100, Daxx) themselves are ISGs and IFN treatment