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viral growth curve; however, once viral production initiates, it proceeds with
normal kinetics to produce near normal levels of progeny (Kudchodkar et al. 2004).
Thus the normal rapamycin sensitivity of mTORC1 had been altered; the delay of
12-24 h in viral growth indicated a period in which the viral infection adapted
to rapamycin. Indeed, the establishment of rapamycin resistance of viral growth
correlated with the development of rapamycin-insensitive phosphorylation of 4E-BP
(Kudchodkar et al. 2004, 2006). Interestingly, the phosphorylation of mTORC1's
other substrate, S6K, does not become rapamycin-insensitive (Kudchodkar et al.
2004, 2006). Thus HCMV's effect on mTOR kinase activity is substrate-specific,
suggesting that HCMV either (1) targets and alters the mTOR complexes directly
for the purpose of facilitating 4E-BP phosphorylation or (2) it induces an mTOR-
independent mechanism for 4E-BP phosphorylation.
Present data suggest that HCMV alters the mTOR complexes. Experiments
using shRNAs to deplete rictor or raptor have shown that HCMV infection alters
the substrate specificity of the rictor-containing complex (Kudchodkar et al. 2006).
Under normal conditions in uninfected cells, only the raptor-containing complex
uses 4E-BP and S6K as substrates; thus depletion of raptor with shRNA results in
a severe inhibition of 4E-BP and S6K phosphorylation while depletion of rictor has
little effect (Sarbassov et al. 2006b). However, in HCMV-infected cells similar
depletion of raptor or rictor caused only about 50% inhibition of 4E-BP and S6K
phosphorylation. This suggests that 4E-BP and S6K can be phosphorylated by
either the rictor- or the raptor-containing complex; this was confirmed by other
experiments (Kudchodkar et al. 2006). Thus the substrate specificity of the rictor-
containing complex is expanded, most likely by structure/function modification,
during HCMV infection.
An additional indication of HCMV-induced structural modification of the rap-
tor- and rictor-containing complexes is the observation that the rapamycin sensitivity
of 4E-BP phosphorylation is altered in infected cells. The raptor-containing complex,
normally sensitive to rapamycin, is insensitive in infected cells; and the rictor-con-
taining complex, normally insensitive for the phosphorylation of Akt, is rapamycin-
sensitive with respect to 4E-BP phosphorylation (Kudchodkar et al. 2006). How the
viral infection mediates these changes remains to be determined; however, virally
induced structural modifications of each complex could account for them. Just as
structural modifications could alter the substrate specificity of the rictor-containing
complexes, they could also change the accessibility of mTOR's FKBP-rapamycin
binding domain such that it becomes accessible in rictor-containing complexes and
occluded in raptor-containing complexes, thus altering rapamycin sensitivity.
HCMV Effects on eIF4E and Mnk-1
While the HCMV-induced phosphorylation of 4E-BP may seem sufficient to allow
eIF4E to be in the eIF4F complex to promote cap-dependent translation, HCMV
maintains an additional mechanism. During HCMV infection, the phosphorylation of
