Identification of cellular proteins interacting with the retroviral restriction factor SAMHD1

Human and mouse SAMHD1 proteins block human immunodeficiency virus type 1 (HIV-1) infection in noncycling human monocytic cells by reducing the intracellular deoxynucleoside triphosphate (dNTP) concentrations. Phosphorylation of human SAMHD1 at threonine 592 (T592) by cyclin-dependent kinase 1 (CDK1) and cyclin A2 impairs its HIV-1 restriction activity, but not the dNTP hydrolase activity, suggesting that dNTP depletion is not the sole mechanism of SAMHD1-mediated HIV-1 restriction. Using coimmunoprecipitation and mass spectrometry, we identified and validated two additional host proteins interacting with human SAMHD1, namely, cyclin-dependent kinase 2 (CDK2) and S-phase kinase-associated protein 2 (SKP2). We observed that mouse SAMHD1 specifically interacted with cyclin A2, cyclin B1, CDK1, and CDK2. Given the role of these SAMHD1-interacting proteins in cell cycle progression, we investigated the regulation of these host proteins by monocyte differentiation and activation of CD4+ T cells and examined their effect on the phosphorylation of human SAMHD1 at T592. Our results indicate that primary monocyte differentiation and CD4+ T-cell activation regulate the expression of these SAMHD1-interacting proteins. Furthermore, our results suggest that, in addition to CDK1 and cyclin A2, CDK2 phosphorylates T592 of human SAMHD1 and thereby regulates its HIV-1 restriction function.

SAMHD1 is the first dNTP triphosphohydrolase found in mammalian cells. Human and mouse SAMHD1 proteins block HIV-1 infection in noncycling cells. Previous studies suggested that phosphorylation of human SAMHD1 at threonine 592 by CDK1 and cyclin A2 negatively regulates its HIV-1 restriction activity. However, it is unclear whether human SAMHD1 interacts with other host proteins in the cyclin A2 and CDK1 complex and whether mouse SAMHD1 shares similar cellular interacting partners. Here, we identify five cell cycle-related host proteins that interact with human and mouse SAMHD1, including three previously unknown cellular proteins (CDK2, cyclin B1, and SKP2). Our results demonstrate that several SAMHD1-interacting cellular proteins regulate phosphorylation of SAMHD1 and play an important role in HIV-1 restriction function. Our findings help define the role of these cellular interacting partners of SAMHD1 that regulate its HIV-1 restriction function.