Ku. Kumar et al., Double-stranded RNA-activated protein kinase (PKR) is negatively regulatedby 60S ribosomal subunit protein L18, MOL CELL B, 19(2), 1999, pp. 1116-1125
The double-stranded RNA (dsRNA)-activated protein kinase (PKR) provides a f
undamental control step in the regulation of protein synthesis initiation t
hrough phosphorylation of the alpha subunit of eukaryotic translation initi
ation factor 2 (eIF-2 alpha), a process that prevents polypeptide chain ini
tiation. In such a manner, activated PKR inhibits cell growth and induces a
poptosis, whereas disruption of normal PKR signaling results in unregulated
cell growth. Therefore, tight control of PKR activity is essential for reg
ulated cell growth. PKR is activated by dsRNA. binding to two conserved dsR
NA binding domains within its amino terminus. We isolated a ribosomal prote
in L18 by interaction with PKR L18 is a 22-kDa protein that is overexpresse
d in colorectal cancer tissue. L18 competed with dsRNA for binding to PKR,
reversed dsRNA binding to PKR, and did not directly bind dsRNA. Mutation of
K64E within the first dsRNA. binding domain of PKR destroyed both dsRNA bi
nding and L18 interaction, suggesting that the two interactive sites overla
p. L18 inhibited both PKR autophosphorylation and PKR-mediated phosphorylat
ion of eIF-2 alpha in vitro. Overexpression of L18 by transient DNA transfe
ction reduced eIF-2 alpha phosphorylation and stimulated translation of a r
eporter gene in vivo. These results demonstrate that L18 is a novel regulat
or of PKR activity and we propose that L18 prevents PKR activation by dsRNA
while PKR is associated with the ribosome, Overexpression of L18 may promo
te protein synthesis and cell growth in certain cancerous tissue through in
hibition of PKR activity.