The control of viral infection by tripartite motif proteins and cyclophilin A

Abstract

The control of retroviral infection by antiviral factors referred to as restriction factors has become an exciting area in infectious disease research. TRIM5alpha has emerged as an important restriction factor impacting on retroviral replication including HIV-1 replication in primates. TRIM5alpha has a tripartite motif comprising RING, B-Box and coiled coil domains. The antiviral alpha splice variant additionally encodes a B30.2 domain which is recruited to incoming viral cores and determines antiviral specificity. TRIM5 is ubiquitinylated and rapidly turned over by the proteasome in a RING dependent way. Protecting restricted virus from degradation, by inhibiting the proteasome, rescues DNA synthesis, but not infectivity, indicating that restriction of infectivity by TRIM5alpha does not depend on the proteasome but the early block to DNA synthesis is likely to be mediated by rapid degradation of the restricted cores. The peptidyl prolyl isomerase enzyme cyclophilin A isomerises a peptide bond on the surface of the HIV-1 capsid and impacts on sensitivity to restriction by TRIM5alpha from Old World monkeys. This suggests that TRIM5alpha from Old World monkeys might have a preference for a particular capsid isomer and suggests a role for cyclophilin A in innate immunity in general. Whether there are more human antiviral TRIMs remains uncertain although the evidence for TRIM19's (PML) antiviral properties continues to grow. A TRIM5-like molecule with broad antiviral activity in cattle suggests that TRIM mediated innate immunity might be common in mammals. Certainly the continued study of restriction of viral infectivity by antiviral host factors will remain of interest to a broad audience and impact on a variety of areas including development of animal models for infection, development of viral vectors for gene therapy and the search for novel antiviral drug targets.

Figure 1

A putative mechanism for restriction of retroviruses by TRIM5α. (Panel A) TRIM5α is autoubiquitinylated in a RING dependent way and rapidly turned over by the proteasome [47]. If it encounters incoming sensitive retroviral cores then they too are recruited to the proteasome and destroyed, before the virus has the opportunity for significant reverse transcription. (Panel B) If the virus/TRIM5α complex is protected from destruction, by inhibiting the proteasome, then the virus can reverse transcribe [48, 49]. Infectivity is not rescued however, indicating that the virus/TRIM5α complex is uninfectious. How TRIM5 renders the virus uninfectious remains unclear.

Figure 2

A putative mechanism for activity of CypA on HIV-1 infectivity in cells from Old World monkeys. HIV-1 recruits CypA to around 10% of its capsid monomers in newly assembled cores [52, 53]. When the core enters the cytoplasm of a target cell it recruits more CypA, which efficiently catalyses cis/trans isomerisation of the peptide bond at CA G89-P90 [42, 57]. This activity replenishes the cis conformation CA as it is recruited into the restricted complex with TRIM5α. If CypA activity is reduced in target cells, using CypA specific siRNA or by inhibiting CypA activity with CSA, then the OWM TRIM5α cannot interact with the CA, which is mostly in the trans conformation, and infectivity is rescued [59-61]. The isomerisation at CA G89-P90 is represented by squares (trans) changing to circles (cis) on the surface of the capsid.

Figure 3

Similarity between the sequences of retroviral capsids. Alignment of primate lentiviral capsid protein sequences demonstrates that they have conserved the proline rich Cyclophilin A binding loop on their outer surface. Glycine proline motifs are common (red arrow). Conserved prolines at the extremes of the loop are shown (black arrows). The alignment from which this selection was taken is available from the Los Alamos HIV sequences database [93]. Retroviruses are named according to the species from which they were isolated. Genbank accession numbers are shown. Species abbreviations are as follows: cpz chimpanzee, deb De Brazza's monkey, den Dent's Mona monkey, drl drill, gsn greater spot nosed monkey, sm sooty mangabey, stm stump tailed macaque, mac rhesus macaque, lst L'Hoest monkey, mnd mandrill, mon Cercopithecus mona, mus Cercopithecus cephus, rcm red capped mangabey, gri African green monkey Grivet, sab African green monkey sabaeus, tan African green monkey tantalus, ver African green monkey vervet, sun sun tailed monkey, syk Sykes monkey.

Figure 4

Similarity between the structures of retroviral capsids. Superimposition of the structures of the N terminal domains of HIV-1 (Red) and MLV (blue) capsids demonstrates overall structural conservation although the Cyclophilin A binding loop (yellow) is absent in MLV. The pdb files for HIV-1 (1M9C) [94] and MLV (1UK7) [40] were superimposed using pairwise structure comparison [95].