Severe Acute and Middle East Respiratory syndrome coronaviruses (SARS-CoV and MERS-CoV) encode multifunctional papain-like proteases (PLPs) that have the ability to process the viral polyprotein to facilitate RNA replication as well as antagonize the host innate-immune response. The latter function involves reversing post-translational modification of cellular proteins conjugated with either ubiquitin (Ub) or Ub-like interferon stimulated gene product 15 (ISG15). Ubiquitin is known to be highly conserved among eukaryotes but surprisingly ISG15 is highly divergent among animals. The ramifications of this sequence divergence to recognition of ISG15 by coronaviral papain-like protease at the structural and biochemical levels are poorly understood. Therefore, the activity of PLPs from SARS-CoV, MERS-CoV and mouse hepatitis virus (MHV) was evaluated against seven ISG15s originating from an assortment of animal species susceptible, and not, to certain coronavirus infections. Excitingly, our kinetic, thermodynamic and structural analysis revealed an array of different preferences among PLPs. Included in these studies is the first insight into a coronoavirus PLP’s interface with ISG15 via SARS-CoV PLP in complex with the principle binding domain of human and mouse ISG15s. The first X-ray structure of the full-length mouse ISG15 protein is also reported and highlights a unique, twisted-hinge region of ISG15 that is not conserved in human ISG15 suggesting a potential role in differential recognition. Taken together, this new information provides a structural and biochemical understanding of the distinct specificities amongst coronavirus PLPs observed and addresses a critical gap of how PLPs can interact with ISG15s from a wide variety of species.

Protein N-myristoylation enables localization to membranes and helps maintain protein conformation and function. N-myristoyltransferases (NMT) catalyze co- or post-translational myristoylation of Src family kinases and other oncogenic proteins, thereby regulating their function. In this study, we provide genetic and pharmacological evidence that inhibiting the N-myristoyltransferase NMT1 suppresses cell cycle progression, proliferation and malignant growth of prostate cancer cells. Loss of myristoylation abolished the tumorigenic potential of Src and its synergy with androgen receptor in mediating tumor invasion. We identified the myristoyl-CoA analog B13 as a small molecule inhibitor of NMT1 enzymatic activity. B13 exposure blocked Src myristoylation and Src localizaiton to the cytoplasmic membrane, attenuating Src-mediated oncogenic signaling. B13 exerted its antiinvasive and antitumor effects against prostate cancer cells with minimal toxic side-effects in vivo. Structural optimization based on structure-activity relationships enabled the chemical synthesis of LCL204 with enhanced inhibitory potency against NMT1. Collectively, our results offer a preclincal proof of concept for the use of protein myristoylation inhibitors as a strategy to block prostate cancer progression.

Background:The type III secretion (T3S) chaperone Scc4 modulates Chlamydia RNA polymerase holoenzyme activity and is also required for secretion of the gatekeeper CopN. Results: Interactions between the Scc4 and Scc1 chaperones and CopN are characterized. Conclusion: Scc4 forms a ternary complex with Scc1 and CopN to promote CopN secretion during infection. Significance: Scc4 is an important link between the T3S system and transcription.

Middle East respiratory syndrome coronavirus (MERS-CoV) is an emerging human pathogen that is the causative agent for Middle East respiratory syndrome (MERS). With MERS outbreaks resulting in over 35% fatalities and now spread to 27 countries, MERS-CoV poses a significant ongoing threat to global human health. As part of its viral genome, MERS-CoV encodes a papain-like protease (PLpro) that has been observed to act as a deubiquitinase and deISGylase to antagonize type I interferon (IFN-I) immune pathways. This activity is in addition to its viral polypeptide cleavage function. Although the overall impact of MERS-CoV PLpro function is observed to be essential, difficulty has been encountered in delineating the importance of its separate functions, particularly its deISGylase activity. As a result, the interface of MERS-CoV and human interferon-stimulated gene product 15 (hISG15) was probed with isothermal calorimetry, which suggests that the C-terminal domain of hISG15 is principally responsible for interactions. Subsequently, the structure of MERS-CoV PLpro was solved to 2.4 Å in complex with the C-terminal domain of hISG15. Utilizing this structural information, mutants were generated that lacked appreciable deISGylase activity but retained wild-type deubiquitinase and peptide cleavage activities. Hence, this provides a new platform for understanding viral deISGylase activity within MERS-CoV and other CoVs. Coronaviruses, such as Middle East respiratory syndrome coronavirus (MERS-CoV), encode a papain-like protease (PLpro) that possesses the ability to antagonize interferon immune pathways through the removal of ubiquitin and interferon-stimulated gene product 15 (ISG15) from target proteins. The lack of CoV proteases with attenuated deISGylase activity has been a key obstacle in delineating the impact between deubiquitinase and deISGylase activities on viral host evasion and pathogenesis. Here, biophysical techniques revealed that MERS-CoV PLpro chiefly engages human ISG15 through its C-terminal domain. The first structure of MERS-CoV PLpro in complex with this domain exposed the interface between these two entities. Employing these structural insights, mutations were employed to selectively remove deISGylase activity with no appreciable impact on its other deubiquitinase and peptide cleavage biochemical properties. Excitingly, this study introduces a new tool to probe the pathogenesis of MERS-CoV and related viruses through the removal of viral deISGylase activity.

Post-translational modification of host and viral proteins by ubiquitin (Ub) and Ub-like proteins, such as interferon stimulated gene product 15 (ISG15), plays a key role in response to infection. Viruses have been increasingly identified that contain proteases possessing deubiquitinase (DUB) and/or deISGylase functions. This includes viruses in the Nairoviridae family that encode a viral homologue of the ovarian tumor protease (vOTU). vOTU activity was recently demonstrated to be critical for replication of the often-fatal Crimean-Congo hemorrhagic fever virus, with DUB activity suppressing the type I interferon responses and deISGylase activity broadly removing ISG15 conjugated proteins. There are currently about 40 known nairoviruses classified into fourteen species. Recent genomic characterization has revealed a high degree of diversity, with vOTUs showing less than 25% amino acids identities within the family. Previous investigations have been limited to only a few closely related nairoviruses, leaving it unclear what impact this diversity has on vOTU function. To probe the effects of vOTU diversity on enzyme activity and specificity, we assessed representative vOTUs spanning the Nairoviridae family towards Ub and ISG15 fluorogenic substrates. This revealed great variation in enzymatic activity and specific substrate preferences. A subset of the vOTUs were further assayed against eight biologically relevant di-Ub substrates, uncovering both common trends and distinct preferences of poly-Ub linkages by vOTUs. Four novel X-ray crystal structures were obtained that provide a biochemical rationale for vOTU substrate preferences and elucidate structural features that distinguish the vOTUs, including a motif in the Hughes orthonairovirus species that has not been previously observed in OTU domains. Additionally, structure-informed mutagenesis provided the first direct evidence of a second site involved in di-Ub binding for vOTUs. These results provide new insight into nairovirus evolution and pathogenesis, and further enhances the development of tools for therapeutic purposes.