The activating NK-cell receptor KIR3DS1 has been implicated in the outcome of various human diseases, including delayed HIV-1 disease progression, yet a ligand that accounts for its biological effects remained unknown. We screened 100 HLA-I proteins and found that KIR3DS1 binds HLA-F, which was validated biochemically and functionally. Primary human KIR3DS1+ NK cells degranulated and produced antiviral cytokines upon encountering HLA-F, and inhibited HIV-1 replication in vitro. CD4+ T-cell activation triggered HLA-F transcription and expression and induced KIR3DS1 ligand expression. HIV-1 infection further increased HLA-F transcription, but decreased KIR3DS1 ligand expression, indicating an immune-evasion mechanism. Altogether, we established HLA-F as a ligand of KIR3DS1, and demonstrated cell-context-dependent expression of HLA-F that may explain the widespread influence of KIR3DS1 in human diseases.

Host proteins are essential for entry and replication of HIV and provide important non-viral therapeutic targets. Large-scale RNAi-based screens have identified nearly a thousand candidate host factors, but with little agreement among studies and few validated factors. Here, we demonstrate that a genome-wide CRISPR-based screen identifies bona fide host factors in a physiologically relevant cell system. We identify five factors, including CD4 and CCR5, that are required for HIV infection yet dispensable for cellular proliferation and viability. TPST2 and SLC35B2 act in a common pathway to sulfate CCR5 on extracellular tyrosine residues, facilitating recognition by HIV envelope. ALCAM mediates cell aggregation, which is required for cell-to-cell HIV transmission. We validate these pathways in primary human CD4+ T cells through Cas9-mediated knockout and antibody blockade. Our findings indicate that HIV infection and replication rely on a limited set of host-dispensable genes and suggest focusing on these pathways for therapeutic intervention.

Heterogeneity of DNA polymerase a [DNA nucleotidyltransferase (DNA-directed), EC 2.7.7.7] has commonly been observed during its purification from a variety of cell systems (1-5). This has hampered the establishment of the physical structure of DNA polymerase a and there is, as yet, no general agreement on the physical properties ofthe core enzyme (6-10). The physiological significance, if any, of this heterogeneity is not clear at this time. Johnston and coworkers (11) showed that mild treatment of a high molecular weight form (200,000-250,000 daltons) of calf thymus DNA polymerase a with 2.5 M urea converts the enzyme to a 155,000-dalton form with the release of a 50,000-to 70,000-dalton protein(s). McKune and Holmes (12) reconstituted the high molecular weight form of polymerase a from the combination of the 155,000-dalton and the 50,000-to 70,000-dalton protein (s). They report that the 50,000-to 70,000-dalton protein(s) enhances the catalytic activity of the polymerase with synthetic polydeoxyribonucleotide templates. Villani et al. (9) have recently shown a similar conversion of DNA polymerase a from Drosophila melanogaster embryos in the presence of2.8 M urea (9). The catalytic activity of the. 148,000-dalton form of DNA polymerase a from Drosophilalembryos (9) and a 156,000-dalton form from rat liver (8) were enhanced when associated with four separable proteins of 54,000-64,000 daltons.We have previously reported the isolationwof a protein (C1) from HeLa cells thattspecifically stimulates the catalytic activity of HeLa DNA polymerase a 20-fold with DNA templateprimers that contain extended single-strand regions (13). In this paper we report the isolation and purification of three forms of DNA polymerase a from synchronized HeLa cells, one ofwhich has equal catalytic activity with activated DNA and with DNA template-primers that contain extended single-strand regions. Two proteins (Cl and C2) that are necessary for its catalytic activity with the latter template-primer are resolved from the DNA polymerase a.MATERIALS AND METHODS Materials. 3H-Labeled deoxyribonucleoside triphosphates were purchased from New England Nuclear. Unlabeled deoxyribonucleoside triphosphates and poly-and oligodeoxyribonucleotide homopolymers were from P-L