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factors similar to Bcl-2 or Bcl-x
L
(Goldmacher et al. 1999). To date, vMIA is the most
broadly antiapoptotic CMV protein known and analogous to the cellular Bcl-2
proteins, is highly effective against a myriad of stimuli including intrinsic stresses as
well as extrinsic, immune-regulated signals (Goldmacher et al. 1999; Belzacq et al.
2001; Vieira et al. 2001; Jan et al. 2002; Roumier et al. 2002; Boya et al. 2003;
Andreau et al. 2004; Arnoult et al. 2004; Boya et al. 2005; McCormick et al. 2005).
However, vMIA does not encode any BH-domains that characterize the cellular pro-
teins (Goldmacher et al. 1999).
vMIA function requires an amino terminal mitochondrial-targeting domain (aa
2-34) and a carboxyl-terminal antiapoptotic domain (AAD, aa 118-147) (Hayajneh
et al. 2001) that together are sufficient for function. The mitochondrial-targeting
domain includes an amino-terminal hydrophobic signal followed by highly con-
served basic residues, and both are required for mitochondrial trafficking
(Mavinakere and Colberg-Poley 2004). Evidence suggests a mitochondrial mem-
brane association with the targeting domain spanning the membrane and the
AAD exposed to the cytoplasm (Mavinakere et al. 2006). The carboxyl-terminal
AAD includes a predicted amphipathic α-helix motif (aa 126-140) critical to
function (Smith and Mocarski 2005). Point mutations predicted to disrupt an
α-helical structure alter amphipathicity or place charge on the hydrophobic face
of the AAD α-helix, each completely abrogate vMIA function. In contrast,
the hydrophilic face of the AAD α-helix tolerates significant substitutions with as
many as five or six amino acid substitutions required to disrupt function (Smith
and Mocarski 2005).
The growth arrest and DNA damage 45 alpha (GADD45α) protein interacts
directly with vMIA in yeast and mammalian cells, fails to bind vMIA mutant pro-
teins, and is essential for vMIA-mediated antiapoptotic activity (Smith and
Mocarski 2005). Targeted knockdown of GADD45α, GADD45β, and GADD45γ
reduced vMIA activity, and each GADD45 family protein individually enhanced
vMIA activity. GADD45α increased both the overall amount of vMIA and that
associated with mitochondrial fractions. Thus, the DNA damage response pathway
is directly linked to vMIA-mediated cell death suppression. Further, vMIA was
shown to bind the antiapoptotic Bcl-2 family protein Bcl-x
L
in mammalian cells.
Collectively, these data suggest that vMIA acts together with Bcl-x
L
and GADD45
to regulate the mitochondrial release of proapoptotic factors (Fig. 2).
In addition to GADD45 proteins, vMIA also binds the proapoptotic Bcl-2
family protein Bax (Arnoult et al. 2004), which has more recently been con-
nected to mitochondrial morphogenesis during life (Karbowski et al. 2006).
In most instances, Bax is distributed in the cytoplasm, but Bax oligomerization
and relocalization to mitochondria mediates the release of proapoptotic factors
from the organelle (Antonsson et al. 2001). In the presence of vMIA, however,
oligomerized Bax at mitochondria fails to promote apoptosis, suggesting
sequestration as a component of the antiapoptotic mechanism (Arnoult et al.
2004; Poncet et al. 2004). Thus, the vMIA-dependent antiapoptotic mechanism
is distinct from that of cellular and viral Bcl-2 proteins that prevent Bax relo-
calization and oligomerization at mitochondria. Recruitment and sequestration
