Truncation of TRIM5 in the Feliformia Explains the Absence of Retroviral Restriction in Cells of the Domestic Cat

Abstract

TRIM5␣ mediates a potent retroviral restriction phenotype in diverse mammalian species. Here, we identify a TRIM5 transcript in cat cells with a truncated B30.2 capsid binding domain and ablated restrictive function which, remarkably, is conserved across the Feliformia. Cat TRIM5 displayed no restriction activity, but ectopic expression conferred a dominant negative effect against human TRIM5␣. Our findings explain the absence of retroviral restriction in cat cells and suggest that disruption of the TRIM5 locus has arisen independently at least twice in the Carnivora, with implications concerning the evolution of the host and pathogen in this taxon.One of the major determinants of host restriction of retroviral replication is the longest (alpha) isoform of the host protein TRIM5, a member of the tripartite motif protein family (34, 42). Human TRIM5␣ (huTRIM5␣) inhibits preintegration stages of murine leukemia virus N-strain (MLV-N) infection, while rhesus macaque TRIM5␣ inhibits human immunodeficiency virus type 1 (HIV-1) infection (13,16,55). Recent studies have demonstrated retroviral restriction by TRIM5␣ species-specific variants from cows (Bos taurus) (41, 56) and rabbits (Oryctolagus cuniculus) (38), suggesting a common ancestor for mammalian TRIM5␣ with antiretroviral properties. Tripartite motif proteins typically comprise a RING domain with E3-ubiquitin ligase activity capable of autoubiquitination (54), a B-box 2 domain, and a coiled-coil domain, collectively know as the RBCC (34). Additionally, some TRIM proteins, including TRIM5␣, possess a C-terminal B30.2 (PRY-SPRY) domain. TRIM5␣ blocks reverse transcription (RT) in most nonpermissive cells (16,42), and evidence suggests that TRIM5␣ homodimers engage the incoming retroviral capsid in the cytoplasm via the B30.2 domain (18,24,40,44). The resulting complex is then degraded rapidly by the proteasome (9, 47) (proteasomal inhibitors restore RT but not viral replication [3,53]). Human splice variants TRIM5␦ and TRIM5␥ lack a B30.2 domain, which disrupts their ability to restrict (30,42). Moreover, these short TRIM5 isoforms have a dominant negative effect, impairing the activity of fulllength TRIM5␣, presumably by the formation of heterodimers (24, 32).

Infectious Entry Pathway Mediated by the Human Endogenous Retrovirus K Envelope Protein

“…We therefore elected to use pseudotypes of the retrovirus HIV-1 to investigate whether HERV-K ENV dictates the same entry requirements for lentiviruses as for VSV. For this purpose, we also selected CRFK cells, as they lack any postentry block to HIV-1 infection (52). The cells were treated with the endocytic inhibitors and infected with HIV-1 pseudotypes bearing VSV-G, HERVK, or HERVK ϩ .…”

Productive Replication of vif -Chimeric HIV-1 in Feline Cells

“…The domestic cat genome encodes a truncated TRIM5␣ protein that lacks the capsid-engaging B30.2 domain and does not restrict HIV-1, SIVmac, or N-tropic murine leukemia virus (N-MLV) (54). Consistent with this, feline cells lack significant postentry restricting activity toward primate lentiviruses and have frequently been used as null backgrounds for testing antiviral properties of primate TRIM5 proteins or for HIV entry receptor studies (34,55).…”

Refrex-1, a Soluble Restriction Factor against Feline Endogenous and Exogenous Retroviruses

“…Host defense against retroviral infection is common, and feline APOBEC and BST-2 are known to function as retroviral restriction factors in cats (21)(22)(23). Interestingly, feline TRIM5 lacks antiretroviral activity (24).…”