TRIM32, which belongs to the tripartite motif (TRIM) protein family, has the RING finger, B-box, and coiled-coil domain structures common to this protein family, along with an additional NHL domain at the C terminus. TRIM32 reportedly functions as an E3 ligase for actin, a protein inhibitor of activated STAT y (PIASy), dysbindin, and c-Myc, and it has been associated with diseases such as muscular dystrophy and epithelial carcinogenesis. Here, we identify a new substrate of TRIM32 and propose a mechanism through which TRIM32 might regulate apoptosis. Our overexpression and knockdown experiments demonstrate that TRIM32 sensitizes cells to TNF␣-induced apoptosis. The RING domain is necessary for this pro-apoptotic function of TRM32 as well as being responsible for its E3 ligase activity. TRIM32 colocalizes and directly interacts with X-linked inhibitor of apoptosis (XIAP), a well known cancer therapeutic target, through its coiled-coil and NHL domains. TRIM32 overexpression enhances XIAP ubiquitination and subsequent proteasomemediated degradation, whereas TRIM32 knockdown has the opposite effect, indicating that XIAP is a substrate of TRIM32. In vitro reconstitution assay reveals that XIAP is directly ubiquitinated by TRIM32. Our novel results collectively suggest that TRIM32 sensitizes TNF␣-induced apoptosis by antagonizing XIAP, an anti-apoptotic downstream effector of TNF␣ signaling. This function may be associated with TRIM32-mediated tumor suppressive mechanism.TRIM32/HT2A is a member of the tripartite motif (TRIM) 5 protein family. To date, 72 TRIM-encoding genes have been identified in the human genome, all sharing the same overall arrangement of RING, B-box, and coiled-coil domains. Their main divergences are in the number of B-boxes and the natures of their C-terminal domains, with most TRIMs possessing one or two additional C-terminal domains, such as B30.2-like/RFP/ SPRY, NHL, ARF, PHD, and BROMO domains (1). TRIM32 has one B-box and six repeats of the NHL motif (2).Several TRIM genes have been associated with disease. For example, mutations in TRIM18/MID1, TRIM20/PYRIN, and TRIM37/MUL have been associated with X-linked Optiz G/BBB syndrome (3), familial Mediterranean fever (4), and mulibrey nanism (5), respectively. Mutations in TRIM32 were recently linked to limb girdle muscular dystrophy type 2H and sarcotubular myopathy, both of which may be caused by the same mutation (D487N) at the third NHL repeat (6). Three additional mutations in the first (R394H), fourth (T520fsX13), and fifth (D588del) NHL repeats of TRIM32 have also been found to cause limb girdle muscular dystrophy type 2H (7). Several TRIM genes have been associated with tumorigenesis; for example, TRIM19/PML (8), TRIM24/TIF1␣ (9, 10), TRIM25/EFP (11), and TRIM27/RFP (12) have been linked to tumor initiation and progression, and the level of TRIM32 mRNA was found to be up-regulated in epidermal cancers (13). However, the precise biological functions of the majority of TRIM family members have not yet been characterized.The RING domain is ...

Sarcopenia is characterized by decreased skeletal muscle mass and function with age. Aged muscles have altered lipid compositions; however, the role and regulation of lipids are unknown. Here we report that FABP3 is upregulated in aged skeletal muscles, disrupting homeostasis via lipid remodeling. Lipidomic analyses reveal that FABP3 overexpression in young muscles alters the membrane lipid composition to that of aged muscle by decreasing polyunsaturated phospholipid acyl chains, while increasing sphingomyelin and lysophosphatidylcholine. FABP3-dependent membrane lipid remodeling causes ER stress via the PERK-eIF2α pathway and inhibits protein synthesis, limiting muscle recovery after immobilization. FABP3 knockdown induces a young-like lipid composition in aged muscles, reduces ER stress, and improves protein synthesis and muscle recovery. Further, FABP3 reduces membrane fluidity and knockdown increases fluidity in vitro, potentially causing ER stress. Therefore, FABP3 drives membrane lipid composition-mediated ER stress to regulate muscle homeostasis during aging and is a valuable target for sarcopenia.

The innate immune response is a host defense mechanism against infection by viruses and bacteria. Type I interferons (IFNα/β) play a crucial role in innate immunity. If not tightly regulated under normal conditions and during immune responses, IFN production can become aberrant, leading to inflammatory and autoimmune diseases. In this study, we identified TRIM11 (tripartite motif containing 11) as a novel negative regulator of IFNβ production. Ectopic expression of TRIM11 decreased IFNβ promoter activity induced by poly (I:C) stimulation or overexpression of RIG-I (retinoic acid-inducible gene-I) signaling cascade components RIG-IN (constitutively active form of RIG-I), MAVS (mitochondrial antiviral signaling protein), or TBK1 (TANK-binding kinase-1). Conversely, TRIM11 knockdown enhanced IFNβ promoter activity induced by these stimuli. Moreover, TRIM11 overexpression inhibited the phosphorylation and dimerization of IRF3 and expression of IFNβ mRNA. By contrast, TRIM11 knockdown increased the IRF3 phosphorylation and IFNβ mRNA expression. We also found that TRIM11 and TBK1, a key kinase that phosphorylates IRF3 in the RIG-I pathway, interacted with each other through CC and CC2 domain, respectively. This interaction was enhanced in the presence of the TBK1 adaptor proteins, NAP1 (NF-κB activating kinase-associated protein-1), SINTBAD (similar to NAP1 TBK1 adaptor) or TANK (TRAF family member-associated NF-κB activator). Consistent with its inhibitory role in RIG-I-mediated IFNβ signaling, TRIM11 overexpression enhanced viral infectivity, whereas TRIM11 knockdown produced the opposite effect. Collectively, our results suggest that TRIM11 inhibits RIG-I-mediated IFNβ production by targeting the TBK1 signaling complex.

Tripartite motif (TRIM) protein TRIM5␣ has been shown to restrict human immunodeficiency virus, type 1 infection in OldWorld monkey cells at the early post-entry step by poorly understood mechanisms. Currently, the physiological function of TRIM5␣ is not known. In this study, we showed that transiently overexpressed TRIM5␣ causes a morphological change in HEK293T cells. A proteomics analysis of the protein complexes that were pulled down with hemagglutinin-tagged TRIM5␣ suggested that the heat shock protein 70 (Hsp70) may serve as a TRIM5␣-binding partner. The interaction between Hsp70 and TRIM5␣ was confirmed by co-localization and co-immunoprecipitation assays. Co-expression of Hsp70 reversed the TRIM5␣-induced morphological change in HEK293T cells. Another heat shock protein Hsc70 also bound to TRIM5␣, but unlike Hsp70, Hsc70 was not able to reverse the TRIM5␣-induced morphological change, suggesting that Hsp70 specifically reverses the morphological change caused by TRIM5␣. Studies using a series of TRIM5␣ deletion mutants demonstrate that, although the PRYSPRY domain is critical for binding to Hsp70, the entire TRIM5␣ structure is necessary to induce the morphological change of cells. When the ATPase domain of Hsp70 was mutated, the mutated Hsp70 could not counteract the morphological change induced by TRIM5␣, indicating that the catalytic activity of Hsp70 protein is important for this function. Co-expression of Hsp70 elevated the levels of TRIM5␣ in the detergent-soluble fraction with a concomitant decrease in the detergent-insoluble fraction. Together these results suggest that Hsp70 plays critical roles in the cellular management against the TRIM5␣-induced cellular insults.Retroviruses encounter dominant post-entry restrictions in the cells of particular species. Human immunodeficiency virus type 1 (HIV-1) 2 infection is blocked in the cells of Old World monkeys and simian immunodeficiency virus (SIV mac ) infection is blocked in most New World monkey cells (1-3). These retroviral restrictions are largely mediated by a host protein, tripartite motif 5␣ (TRIM5␣) (4). TRIM5␣ is a member of the large family of tripartite motif proteins (TRIMs) (5). Members of this family commonly have a RING domain, one or two B-box domains and coiled-coil (CC) domain, and thus are also called RBCC proteins (5). The RING domain of many TRIM proteins has been shown to have E3 ubiquitin ligase activity, whereas the B box and CC domains may be involved in protein-protein interactions and homo/heterodimerization (5). The alpha isoforms of many cytoplasmic TRIM proteins, including TRIM5␣, contain the PRYSPRY domain whose function is unknown. To date, about 70 TRIM proteins have been identified in the human genome and some of their homologs have been found in primates and other species. TRIM proteins arose with the metazoans and have expanded in number during vertebrate evolution (5). Many TRIM proteins, including TRIM19, TRIM21, TRIM22, TRIM34, and TRIM5␣, can be up-regulated by interferon, supporting their potential role as ef...