MX2-mediated innate immunity against HIV-1 is regulated by serine phosphorylation

Abstract

The antiviral cytokine interferon (IFN) activates expression of IFN-stimulated genes (ISGs) to establish an antiviral state. Myxovirus resistance 2 (MX2/MxB) is an ISG that inhibits the nuclear import of HIV-1 and interacts with the viral capsid and cellular nuclear transport machinery. We identified the myosin light chain phosphatase (MLCP) subunits MYPT1 and PPP1CB as positively-acting regulators of MX2, interacting with its N-terminal domain (NTD). We demonstrated that serine phosphorylation of the NTD at positions 14, 17 and 18 suppresses MX2 antiviral function, prevents interactions with the HIV-1 capsid and nuclear transport factors, and is reversed by MLCP. Importantly, NTD serine phosphorylation also impedes MX2-mediated inhibition of nuclear import of cellular karyophilic cargo. We additionally found that IFN treatment reduces levels of phosphorylation at these serines and outline a homeostatic regulatory mechanism where repression of MX2 by phosphorylation, together with MLCP-mediated dephosphorylation, balances the deleterious effects of MX2 upon normal cell function with innate immunity against HIV-1.

Host variation in type I interferon signaling genes (MX1),CCR5Δ32, and MHC class I alleles in treated HIV+ non-controllers predict viral reservoir size

“…We also observed two additional SNPs significantly associated with HIV tDNA, the first in PPP1CB (p=1.5×10 −7 , q=0.03), which encodes Protein Phosphatase 1 Catalytic Subunit Beta, a protein shown to reduce the antiviral potency of MX2 against HIV-1 (58), and LRMP (p=4.1×10 −6 , q=0.03), which encodes Lymphoid-Restricted Membrane Protein, which plays a critical role in the delivery of peptides to MHC class I molecules (61) ( Table 1 ). Additional SNPs that showed trends with HIV tDNA included: DDX3X (p=2.5×10 −5 , q=0.17), which encodes DEAD-box helicase 3 X-linked, a protein that regulates the production of type I interferons (62), and AKAP6 (p=3.0×10 −5 , q=0.20), encoding A-Kinase Anchoring Protein 6, which that binds to regulatory subunits of protein kinase A, a critical signaling pathway that has been associated with HIV latency reversal and T cell proliferation (63, 64).…”

“…The most striking finding from our SNP-based analysis was the identification of several SNPs in the human interferon-inducible myxovirus resistance 1 gene, MX1, also known as MXA, as well as additional genes linked to type I interferon signaling ( Table 1, Table S7 ). The encoded protein, MX1, is closely related to MX2 (MXB), a potent host restriction factor that inhibits HIV infection prior to integration (56–58). MX1 itself is long recognized as a potent antiviral factor, inhibiting replication of several RNA viruses, including influenza A and measles viruses, and DNA viruses, such as hepatitis B virus (60).…”

“…We also observed statistically significant ( PPP1CB ) and suggestive ( DDX3X ) associations with HIV tDNA and these additional interferon-related genes. PPP1CB encodes Protein Phosphatase 1 Catalytic Subunit Beta, which regulates the antiviral potency of MX2 (58). PP1CB has been shown to interact with HIV Tat and is required for Tat-induced HIV-1 transcription in cultured cells; suppression of PP1 has been shown to inhibit HIV-1 transcription in cell lines (80).…”

Schlafens Can Put Viruses to Sleep

“…No evidence has yet been found to support the hypothesis that viruses regulate the phosphorylation of SLFN11 through virus-encoded kinases, or indirectly through host cell kinases, such as PPP1CC. Further research is required to explore the possibility that viruses exploit protein phosphorylation to circumvent the antiviral activity of Schlafens, as has been observed for other host restriction factors [105][106][107][108][109][110][111][112].…”

Regulation of Viral Restriction by Post-Translational Modifications

“…MxB activity is negatively regulated through its phosphorylation [169]. Three sites of phosphorylation have been identified on serines at positions 14, 17 and 18 on the N-terminal domain of MxB (Figure 2).…”