Responsible for the ongoing coronavirus disease 19 (COVID-19) pandemic, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infects host cells through binding of the viral spike protein (SARS-2-S) to the cell-surface receptor angiotensin-converting enzyme 2 (ACE2). Here we show that the high-density lipoprotein (HDL) scavenger receptor B type 1 (SR-B1) facilitates ACE2-dependent entry of SARS-CoV-2. We find that the S1 subunit of SARS-2-S binds to cholesterol and possibly to HDL components to enhance viral uptake in vitro. SR-B1 expression facilitates SARS-CoV-2 entry into ACE2-expressing cells by augmenting virus attachment. Blockade of the cholesterol-binding site on SARS-2-S1 with a monoclonal antibody, or treatment of cultured cells with pharmacological SR-B1 antagonists, inhibits HDL-enhanced SARS-CoV-2 infection. We further show that SR-B1 is coexpressed with ACE2 in human pulmonary tissue and in several extrapulmonary tissues. Our findings reveal that SR-B1 acts as a host factor that promotes SARS-CoV-2 entry and may help explain viral tropism, identify a possible molecular connection between COVID-19 and lipoprotein metabolism, and highlight SR-B1 as a potential therapeutic target to interfere with SARS-CoV-2 infection.

Material Supplementary 4.DC1http://www.jimmunol.org/content/suppl/2010/06/16/jimmunol.090387

Stringent control of inflammasome signaling pathway is important for maintaining immunological balance, yet the molecular mechanisms responsible for its tight regulation are still poorly understood. In this study, we found that the signaling pathway dependent on mitochondrial antiviral signaling protein (MAVS) was required for the optimal activation of apoptosis-associated specklike protein (ASC)–dependent inflammasome. In particular, TNFR-associated factor 3 was found to be a direct E3 ligase for ASC. Ubiquitination of ASC at Lys174 was critical for speck formation and inflammasome activation. Deficiency in MAVS or TNFR-associated factor 3 impaired ASC ubiquitination and cytosolic aggregates formation, resulting in reduced inflammasome response upon RNA virus infection. This study has identified a previously unrecognized role of MAVS in the regulation of inflammasome signaling and provided molecular insight into the mechanisms by which ubiquitination of ASC controls inflammasome activity through the formation of ASC specks.

Viral infection triggers the formation of mitochondrial antiviral signaling protein ( MAVS ) aggregates, which potently promote immune signaling. Autophagy plays an important role in controlling MAVS ‐mediated antiviral signaling; however, the exact molecular mechanism underlying the targeted autophagic degradation of MAVS remains unclear. Here, we investigated the mechanism by which RNF 34 regulates immunity and mitophagy by targeting MAVS . RNF 34 binds to MAVS in the mitochondrial compartment after viral infection and negatively regulates RIG ‐I‐like receptor ( RLR )‐mediated antiviral immunity. Moreover, RNF 34 catalyzes the K27‐/K29‐linked ubiquitination of MAVS at Lys 297, 311, 348, and 362 Arg, which serves as a recognition signal for NDP 52‐dependent autophagic degradation. Specifically, RNF 34 initiates the K63‐ to K27‐linked ubiquitination transition on MAVS primarily at Lys 311, which facilitates the autophagic degradation of MAVS upon RIG ‐I stimulation. Notably, RNF 34 is required for the clearance of damaged mitochondria upon viral infection. Thus, we elucidated the mechanism by which RNF 34‐mediated autophagic degradation of MAVS regulates the innate immune response, mitochondrial homeostasis, and infection.

Innate immunity to viruses involves receptors such as Retinoic Acid Induced Gene-1 (RIG-I), which senses viral RNA and triggers a signaling pathway involving the outer mitochondrial membrane protein mitochondrial antiviral signaling (MAVS). Recent work has identified that NLRX1, a member of another class of innate immune receptors, sequesters MAVS away from RIG-I and thereby prevents mitochondrial antiviral immunity. In this study, we demonstrate that the proteasome PSMA7 (α4) subunit associates with MAVS in vivo and in vitro. Expression of PSMA7 results in a potent inhibition of RIG-1 and MAVS-mediated IFN-β promoter activity; conversely, depletion of PSMA7 with small interference RNA enhances virus-induced type I IFN production, with consequent reduction of virus replication. Furthermore, a striking reduction in the abundance of endogenous MAVS with overexpressed PSMA7 was found and virus infection leads to transient increase in the endogenous PSMA7 protein level. Cumulatively, these results suggest that PSMA7 is a negative regulator of the MAVS-mediated innate immunity that probably serves to attenuate the establishment of an antiviral state during viral infection, highlighting the biological significance of PSMA7-MAVS association as an important cellular regulatory control.