Phosphorylation of MX2 regulates innate immunity against HIV-1

Abstract Interferon (IFN) mobilizes a cellular anti-viral state by inducing the expression of IFN-stimulated genes (ISGs). Myxovirus resistance 2 (MX2/MxB) is an ISG that inhibits HIV-1 infection by suppressing viral import into the nucleus. The amino-terminal domain (NTD) of MX2 plays an essential role in viral inhibition and, here, we exploit proteomic screening to identify the myosin light chain phosphatase (MLCP) components MYPT1 and PPP1CB as key NTD binding partners. Experimental depletion of either protein, or pharmacologic inhibition of MLCP, reduces MX2 anti-viral activity. Analysis of post-translational modifications defined sites of phosphorylation within the NTD, and replacement of the serines at positions 14, 17 and 18 with the phosphomimetic aspartic acid phenocopies MLCP silencing by abrogating anti-viral function. Phosphorylation of this triple-serine motif impedes MX2’s interaction with its HIV-1 protein target, Capsid, reduces MX2 accumulation at the nuclear envelope, and weakens MX2’s ability to inhibit the nuclear import of non-viral karyophilic cargo. Finally, IFN itself acts to reduce levels of phosphorylation at these key regulatory serines. We therefore propose that homeostatic repression of MX2 through NTD phosphorylation, together with its reversal by MLCP and IFN, balances the deleterious effects of MX2 on normal cell function with innate immunity against HIV-1.