SUMOylation of SAMHD1 at Lysine 595 is required for HIV-1 restriction in non-cycling cells

Abstract

SAMHD1 is a cellular triphosphohydrolase (dNTPase) proposed to inhibit HIV-1 reverse transcription in non-cycling immune cells by limiting the supply of the dNTP substrates. Yet, phosphorylation of T592 downregulates SAMHD1 antiviral activity, but not its dNTPase function, implying that additional mechanisms contribute to viral restriction. Here, we show that SAMHD1 is SUMOylated on residue K595, a modification that relies on the presence of a proximal SUMO-interacting motif (SIM). Loss of K595 SUMOylation suppresses the restriction activity of SAMHD1, even in the context of the constitutively active phospho-ablative T592A mutant but has no impact on dNTP depletion. Conversely, the artificial fusion of SUMO to a non-SUMOylatable inactive SAMHD1 variant restores its antiviral function. These observations clearly establish that the absence of T592 phosphorylation cannot fully account for the restriction activity of SAMHD1. We find that concomitant SUMOylation of K595 is required to stimulate a dNTPase-independent antiviral activity.