The rhesus monkey intrinsic immunity factor TRIM5␣ rh recognizes incoming capsids from a variety of retroviruses, including human immunodeficiency virus type 1 (HIV-1) and equine infectious anemia virus (EIAV), and inhibits the accumulation of viral reverse transcripts. However, direct interactions between restricting TRIM5␣ proteins and retroviral capsids have not previously been demonstrated using pure recombinant proteins. To facilitate structural and mechanistic studies of retroviral restriction, we have developed methods for expressing and purifying an active chimeric TRIM5␣ rh protein containing the RING domain from the related human TRIM21 protein. This recombinant TRIM5-21R protein was expressed in SF-21 insect cells and purified through three chromatographic steps. Two distinct TRIM5-21R species were purified and shown to correspond to monomers and dimers, as analyzed by analytical ultracentrifugation. Chemically cross-linked recombinant TRIM5-21R dimers and mammalian-expressed TRIM5-21R and TRIM5␣ proteins exhibited similar sodium dodecyl sulfate-polyacrylamide gel electrophoresis mobilities, indicating that mammalian TRIM5␣ proteins are predominantly dimeric. Purified TRIM5-21R had ubiquitin ligase activity and could autoubquitylate with different E2 ubiquitin conjugating enzymes in vitro. TRIM5-21R bound directly to synthetic capsids composed of recombinant HIV-1 CA-NC proteins and to authentic EIAV core particles. HIV-1 CA-NC assemblies bound dimeric TRIM5-21R better than either monomeric TRIM5-21R or TRIM5-21R constructs that lacked the SPRY domain or its V1 loop. Thus, our studies indicate that TRIM5␣ proteins are dimeric ubiquitin E3 ligases that recognize retroviral capsids through direct interactions mediated by the SPRY domain and demonstrate that these activities can be recapitulated in vitro using pure recombinant proteins.Susceptibility to retroviral infections influences species survival and has driven the evolution of cellular restriction factors that inhibit retroviral replication. One important antiretroviral intrinsic immune response is mediated by TRIM5␣, which can block early postentry steps in the replication of certain retroviruses in specific primate lineages (3, 37, 55, 58). Under normal restrictive conditions, TRIM5␣ proteins block accumulation of retroviral reverse transcripts (55) and accelerate the rate at which viral capsids dissociate from high-molecularweight complexes into lower-molecular-weight subunits (42,56). The allelic specificity of TRIM5␣ restriction is illustrated by the fact that rhesus macaque TRIM5␣ potently inhibits human immunodeficiency virus type 1 (HIV-1) replication, whereas human TRIM5␣ instead exhibits restriction activity against N-tropic murine leukemia virus but not 29,43,55,67). These differences can be attributed to the differential abilities of TRIM5␣ proteins to interact with retroviral capsids after viral entry (34,42,56).Like other tripartite (TRIM) family members, TRIM5␣ contains RING, B-box, coiled-coil, and B30.2/SPRY domains, and each of t...

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The restriction factors, TRIM5alpha in most primates and TRIMCyp in owl monkeys, block infection of various retroviruses soon after virus entry into the host cell. Rhesus monkey TRIM5alpha (TRIM5alpha rh) inhibits human immunodeficiency virus (HIV-1) and feline immunodeficiency virus (FIV) more potently than human TRIM5alpha (TRIM5alpha hu). TRIMCyp restricts infection of HIV-1, simian immunodeficiency virus of African green monkeys (SIV agm) and FIV. Early after infection, TRIMCyp, like TRIM5alpha rh and TRIM5alpha hu, decreased the amount of particulate viral capsid in the cytosol of infected cells. The requirements for the TRIMCyp and TRIM5alpha domains in restricting different retroviruses were investigated. Potent restriction of FIV by TRIMCyp occurred in the complete absence of RING and B-box 2 domains; by contrast, efficient FIV restriction by TRIM5alpha rh required these domains. Variable region 1 of the TRIM5alpha rh B30.2 domain contributed to the potency of HIV-1, FIV and equine infectious anemia virus restriction. Thus, although differences exist in the requirements of TRIMCyp and TRIM5alpha for RING/B-box 2 domains, both restriction factors exhibit mechanistic similarities.