The zinc containing pro-apoptotic protein siva interacts with the peroxisomal membrane protein pmp22

The N-terminal 33 amino acid domain of Siva-1 is sufficient for nuclear localization

Siva-1 induces apoptosis in multiple pathological processes and plays an important role in the suppression of tumor metastasis, protein degradation, and other functions. Although many studies have demonstrated that Siva-1 functions in the cytoplasm, a few have found that Siva-1 can relocate to the nucleus. In this study, we found that the first 33 amino acid residues of Siva-1 are required for its nuclear localization. Further study demonstrated that the green fluorescent protein can be imported into the nucleus after fusion with these 33 amino acid residues. Other Siva-1 regions and domains showed less effect on Siva-1 nuclear localization. By site-mutagenesis of all of these 33 amino acid residues, we found that mutants of the first 1-18 amino acids affected Siva-1 nuclear compartmentalization but could not complete this localization independently. In summary, we demonstrated that the N-terminal 33 amino acid residues were sufficient for Siva-1 nuclear localization, but the mechanism of this translocation needs additional investigation.

The PMP22 gene and its related diseases

Peripheral myelin protein-22 (PMP22) is primarily expressed in the compact myelin of the peripheral nervous system. Levels of PMP22 have to be tightly regulated since alterations of PMP22 levels by mutations of the PMP22 gene are responsible for >50 % of all patients with inherited peripheral neuropathies, including Charcot-Marie-Tooth type-1A (CMT1A) with trisomy of PMP22, hereditary neuropathy with liability to pressure palsies (HNPP) with heterozygous deletion of PMP22, and CMT1E with point mutations of PMP22. While overexpression and point-mutations of the PMP22 gene may produce gain-of-function phenotypes, deletion of PMP22 results in a loss-of-function phenotype that reveals the normal physiological functions of the PMP22 protein. In this article, we will review the basic genetics, biochemistry and molecular structure of PMP22, followed by discussion of the current understanding of pathogenic mechanisms involving in the inherited neuropathies with mutations in PMP22 gene.

Regulation of IL-2 gene expression by Siva and FOXP3 in human T cells

BACKGROUND

Severe autoinflammatory diseases are associated with mutations in the Foxp3 locus in both mice and humans. Foxp3 is required for the development, function, and maintenance of regulatory T cells (Tregs), a subset of CD4 cells that suppress T cell activation and inflammatory processes. Siva is a pro-apoptotic gene that is expressed across a range of tissues, including CD4 T cells. Siva interacts with three tumor necrosis factor receptor (TNFR) family members that are constitutively expressed on Treg cells: CD27, GITR, and OX40.

RESULTS

Here we report a biophysical interaction between FOXP3 and Siva. We mapped the interaction domains to Siva's C-terminus and to a central region of FOXP3. We showed that Siva repressed IL-2 induction by suppressing IL-2 promoter activity during T cell activation. Siva-1's repressive effect on IL-2 gene expression appears to be mediated by inhibition of NFkappaB, whereas FOXP3 repressed both NFkappaB and NFAT activity.

CONCLUSIONS

In summary, our data suggest that both FOXP3 and Siva function as negative regulators of IL-2 gene expression in Treg cells, via suppression of NFAT by FOXP3 and of NFkappaB by both FOXP3 and Siva. Our work contributes evidence for Siva's role as a T cell signalling mediator in addition to its known pro-apoptotic function. Though further investigations are needed, evidence for the biophysical interaction between FOXP3 and Siva invites the possibility that Siva may be important for proper Treg cell function.

Siva1 is a XIAP-interacting protein that balances NFkappaB and JNK signalling to promote apoptosis

XIAP is known as a potent inhibitor of apoptosis, but in addition is involved in cellular signalling, including the NFkappaB, JNK and TGFbeta pathways. Our search for XIAP-interacting partners led us to Siva1, a proapoptotic protein that is known to play a role in T-cell apoptosis through a caspase-dependent mitochondrial pathway. The interaction sites between XIAP and Siva1 were mapped to the RING domain of XIAP and the N-terminal, SAH-containing and death-homology-region-containing domains of Siva1. Co-immunoprecipitation experiments showed that XIAP, Siva1 and TAK1 form a ternary complex in Jurkat T cells. Reporter-gene analysis revealed that Siva1 inhibits XIAP- and TAK1-TAB1-mediated NFkappaB activation. By contrast, Siva1 increased XIAP- and TNFalpha-mediated AP1 activity and prolonged TNFalpha-induced JNK activation, whereas knock down of Siva1 resulted in reduced JNK activation. This suggests that Siva1 differentially modulates signalling by JNK and NFkappaB and shifts the balance between these pathways towards enhanced JNK activation, a situation that promotes apoptosis. Ectopically expressed Siva1 increased caspase-3 activity, which was inhibited by XIAP in a ubiquitin-ligase-dependent manner. In line with this, Siva1 was lysine-48-linked polyubiquitylated by XIAP. Our findings suggest that, via physical interaction with XIAP and TAK1, Siva1 diminishes NFkappaB and enhances JNK activity to favour apoptosis.

Proapoptotic protein Siva binds to the muscle protein telethonin in cardiomyocytes during coxsackieviral infection

AIMS

Coxsackievirus B3 (CVB3) is known to cause a variety of human diseases including acute and chronic myocarditis as well as dilated cardiomyopathy (DCM). However, the mechanisms by which CVB3 causes diseases are not well understood.

METHODS AND RESULTS

Studies identifying protein-protein interactions during CVB3 infection are useful in delineating the pathogenesis of acute or chronic myocarditis. Screening a human heart cDNA library revealed a yet unknown interaction partner of the proapoptotic protein Siva. We demonstrate that Siva specifically interacts with the heart and skeletal muscle protein telethonin. The expression of Siva is increased in heart tissue of CVB3-infected mice and the proteins colocalize in cardiomyocytes.

CONCLUSION

telethonin might be involved in CVB3-mediated cell damage and in the resulting cardiac dysfunction due to the interaction with Siva. We suggest a molecular mechanism through which coxsackieviral infection contributes to the pathogenesis of chronic myocarditis and in particular of acquired forms of DCM.

Pyrin, product of the MEFV locus, interacts with the proapoptotic protein, Siva

Mutations in pyrin cause the autoinflammatory disorder familial Mediterranean fever (FMF), a syndrome characterized by sporadic and unpredictable attacks of fever and localized severe pain. Currently, it is not clear how attacks are triggered, nor why they spontaneously resolve after 2 or 3 days. In fact, the cellular function of the pyrin protein and the molecular underpinnings of its malfunction in FMF have so far eluded clear definition. The identification of pyrin-interacting proteins has the potential to increase our understanding of the cellular networks in which pyrin functions. Previous reports have established that pyrin interacts with the apoptotic protein ASC, the cytoskeletal adaptor protein PSTPIP1, the inflammatory caspase, Caspase-1 and certain forms of the cytosolic anchoring protein 14-3-3. Here, we report that pyrin also interacts with Siva, a pro-apoptotic protein first identified for its interaction with the cytosolic tail of CD27, a TNF family receptor. The interaction between pyrin and Siva involves the C-terminal B30.2/rfp/SRPY domain of pyrin and exon 1 of Siva. We show that Siva and pyrin are indeed co-expressed in human neutrophils, monocytes, and synovial cells. Furthermore, using a novel protein/protein interaction assay, we demonstrate that pyrin can recruit Siva to ASC specks, establishing a potential platform for intersection of ASC and Siva function. Finally, we show that pyrin modulates the apoptotic response to oxidative stress mediated by Siva. Thus, the Siva-pyrin interaction may be a potential target for future therapeutic strategies.