Phosphorylation of mouse SAMHD1 regulates its restriction of human immunodeficiency virus type 1 infection, but not murine leukemia virus infection.

Human SAMHD1 (hSAMHD1) restricts HIV-1 infection in non-dividing cells by depleting intracellular dNTPs to limit viral reverse transcription. Phosphorylation of hSAMHD1 at threonine (T) 592 by cyclin-dependent kinase (CDK) 1 and CDK2 negatively regulates HIV-1 restriction. Mouse SAMHD1 (mSAMHD1) restricts HIV-1 infection in non-dividing cells, but whether its phosphorylation regulates retroviral restriction is unknown. Here we identified six phospho-sites of mSAMHD1, including T634 that is homologous to T592 of hSAMHD1 and phosphorylated by CDK1 and CDK2. We found that wild-type (WT) mSAMHD1 and a phospho-ablative mutant, but not a phospho-mimetic mutant, restricted HIV-1 infection in differentiated U937 cells. Murine leukemia virus (MLV) infection of dividing NIH3T3 cells was modestly restricted by mSAMHD1 WT and phospho-mutants, but not by a dNTPase-defective mutant. Our results suggest that phosphorylation of mSAMHD1 at T634 by CDK1/2 negatively regulates its HIV-1 restriction in differentiated cells, but does not affect its MLV restriction in dividing cells.