Modulation of TRIM5α Activity in Human Cells by Alternatively Spliced TRIM5 Isoforms

Abstract

TRIM5␣ is a restriction factor that can block an early step in the retroviral life cycle by recognizing and causing the disassembly of incoming viral capsids, thereby preventing the completion of reverse transcription. Numerous other isoforms of human TRIM5 exist, and isoforms lacking a C-terminal SPRY domain can inhibit the activity of TRIM5␣. Thus, TRIM5␣ activity in a given cell type could be dependent on the relative proportions of TRIM5 isoforms expressed, but little information concerning the relative expression of TRIM5 isoforms in human cells is available. In this study, we demonstrate that mRNAs coding for TRIM5␣ represent only 50% of total TRIM5 transcripts in human cell lines, CD4 ؉ T cells, and macrophages. Transcripts coding for, in order of abundance, TRIM5 (TRIM5-iota), a previously uncharacterized isoform, TRIM5␥, TRIM5␦, and TRIM5 are also present. Like TRIM5␥ and TRIM5␦, TRIM5 and TRIM5 do not inhibit HIV-1 replication, but both have dominant-negative activity against TRIM5␣. Specific knockdown of TRIM5 increases TRIM5␣ activity in human U373-X4 cells, indicating that physiological levels of expression of truncated TRIM5 isoforms in human cells can reduce the activity of TRIM5␣.Following the entry of retroviral capsids into the cytoplasm of target cells, they can encounter cellular restriction factors that block the early steps of the viral life cycle. One wellcharacterized early restriction factor is TRIM5␣ (22,27,41,46), whose recognition of the incoming capsid leads to rapid capsid disassembly, preventing the completion of reverse transcription (42). Human TRIM5␣ can potently inhibit N-tropic murine leukemia virus (N-MLV) (12,18,31,48) and moderately inhibit equine infectious anemia virus and feline immunodeficiency virus replication (8,18,33). The infectivity of laboratory-adapted strains of HIV-1 is inhibited only 2-to 3-fold by the physiological levels of human TRIM5␣ expressed in human cells (13,17,39,44,49), but HIV-1 expressing capsid proteins derived from clinical isolates can be considerably more sensitive to human TRIM5␣, especially if TRIM5␣ expression in target cells is augmented by pretreatment with alpha interferon (IFN-␣) (1).TRIM5␣ is the longest of numerous TRIM5 isoforms expressed in human cells (Fig. 1) and is composed of several distinct domains. The RING domain (coded by exon 2) expresses E3 ubiquitin ligase activity, is required for optimal antiviral activity, and contributes to the rapid turnover of TRIM5␣ (9,15,41). The function of the B-box 2 domain (coded by exon 2) is not fully understood, but amino acid changes in this region can influence TRIM5␣ turnover, higherorder self-association of TRIM5␣ dimers, the formation of TRIM5␣-containing cytoplasmic bodies, and antiviral activity (7,10,15,20). The coiled-coil domain (coded by exons 2 to 4) promotes the formation of homodimers and participates in capsid recognition (16,19,24,26,30). These three domains comprise the RING/B-box/coiled-coil (RBCC) tripartite motif characteristic of all TRIM proteins. Finally, TRIM5␣ als...

Stabilized Human TRIM5α Protects Human T Cells From HIV-1 Infection

“…17,[32][33][34] Most have interpreted these data to mean that huTRIM5α has a reduced ability to recognize and restrict HIV- huTRIM5α R323-332 construct enable potent HIV-1 restriction, it has been postulated that these changes to huTRIM5α augment its affinity for capsid. However, our data show that these changes also augment TRIM5α stability.…”

Pressure from TRIM5α Contributes to Control of HIV-1 Replication by Individuals Expressing Protective HLA-B Alleles

“…Variation in hTRIM5␣ expression has been described (84), possibly reflecting polymorphisms in transcription factor-binding sites (85). In addition, overall hTRIM5␣ activity can be influenced by its induction by IFN-␣ (44,86,87) and the relative expression of hTRIM5␣ and the inhibitory hTRIM5 splicing variants (gamma, delta, iota) (88). Furthermore, several hTRIM5␣ allelic variants have been identified with impaired activity, including those carrying the H43Y and G249D polymorphisms, although both the effect of these polymorphisms on TRIM5␣ activity and the impact of expression of these variants on disease progression remain controversial (44,83,(89)(90)(91)(92)(93)(94)(95)(96)(97).…”

Ovine TRIM5α Can Restrict Visna/Maedi Virus

“…Some of these lack the PRYSPRY domain and are therefore unrestrictive. Like human TRIM5␥ (36), the short ovine isoform is likely to act as a dominant negative through lack of a viral binding PRYSPRY domain (2).…”