An Anti-apoptotic Viral Protein That Recruits Bax to Mitochondria

Abstract

The viral mitochondria-localized inhibitor of apoptosis (vMIA), encoded by the UL37 gene of human cytomegalovirus, inhibits apoptosis-associated mitochondrial membrane permeabilization by a mechanism different from that of Bcl-2. Here we show that vMIA induces several changes in Bax that resemble those found in apoptotic cells yet take place in unstimulated, nonapoptotic vMIA-expressing cells. These changes include the constitutive localization of Bax at mitochondria, where it associates tightly with the mitochondrial membrane, forming high molecular weight aggregates that contain vMIA. vMIA recruits Bax to mitochondria but delays relocation of caspase-8-activated truncated Bidgreen fluorescent protein (GFP) (t-Bid-GFP) to mitochondria. The ability of vMIA and its deletion mutants to associate with Bax and to induce relocation of Bax to mitochondria correlates with their anti-apoptotic activity and with their ability to suppress mitochondrial membrane permeabilization. Taken together, our data indicate that vMIA blocks apoptosis via its interaction with Bax. vMIA neutralizes Bax by recruiting it to mitochondria and "freezing" its pro-apoptotic activity. These data unravel a novel strategy of subverting an intrinsic pathway of apoptotic signaling.Apoptosis is mediated through two main pathways, the extrinsic (death receptor) pathway and the intrinsic (mitochondrial) pathway. The extrinsic pathway is initiated by ligation of a plasma membrane death receptor, which results in a stepwise recruitment of adaptors and initiator caspases (in particular, caspase-8) into the death-inducing signaling complex.

Mitochondria as therapeutic targets for cancer chemotherapy

“…Bax combined with another proapoptotic protein of the Bcl-2 family, t-Bid, can permeabilize cardiolipin-containing liposomes in vitro to fluorescent dextran of size up to 2000 kDa (Kuwana et al, 2002), correlating with the fact that apoptotic MOMP occurring in true cells is nonselective and affects all soluble intermembrane proteins, irrespective of their size (Patterson et al, 2000). The mechanisms of membrane permeabilization induced in isolated mitochondria exposed to recombinant Bax depends on its oligomerization status, although oligomerization by itself may be insufficient for the induction of MOMP (Poncet et al, 2004). Bax oligomers trigger cyclosporin A (CsA)-resistant cyt c release, while Bax monomers cause permeabilization through a pathway that can be inhibited by CsA (Gogvadze et al, 2001).…”

Mitochondrial and Secretory Human Cytomegalovirus UL37 Proteins Traffic into Mitochondrion-Associated Membranes of Human Cells

“…These sequences include a strongly hydrophobic leader (aa 1 to 22) and a juxtaposed basic domain (aa 23 to 34), which jointly serve as a bipartite signal to target UL37 proteins to the ER and mitochondria (25,27). This NH 2 -terminal targeting domain constitutes the first UL37x1 antiapoptotic domain, which when combined with a downstream domain (aa 118 to 147) are sufficient to confer its antiapoptotic activity (5,18,19,28,32,34,45). Between the UL37x1 antiapoptotic domains lies an acidic domain (aa 81 to 108), which plays a role in the transactivation of HCMV early gene promoters, whose products are needed for its oriLyt replication (13,31,52).…”

HIV-1 protease inhibitors and cytomegalovirus vMIA induce mitochondrial fragmentation without triggering apoptosis

“…In a further series of experiments, we investigated the effect of the cytomegalovirus early gene product viral mitochondrial inhibitor of apoptosis (vMIA) 27,28 (a product of the UL37 exon1 gene) on mitochondrial morphology. vMIA induced a marked shift from a tubular to a punctuate mitochondrial distribution (McCormick et al 29 and Supplementary Figure 1a), as detectable in normal HeLa cells.…”

“…6,7 vMIA, which induces marked mitochondrial fragmentation, is -paradoxically -a rather potent, general inhibitor of MOMP and apoptosis. 27,28,31 vMIA inhibits MOMP, presumably by recruiting Bax to mitochondria while antagonizing its pore-forming function. 28,32 However, vMIA can induce mitochondrial fragmentation in cells that lack Bax expression (not shown), indicating that the precise mechanism through which vMIA influences mitochondrial dynamics are elusive.…”