REGULATION OF P53-DEPENDENT APOPTOSIS, TRANSCRIPTIONAL REPRESSION, AND CELL-TRANSFORMATION BY PHOSPHORYLATION OF THE 55-KILODALTON E1B PROTEIN OF HUMAN ADENOVIRUS TYPE-5
Jg. Teodoro et Pe. Branton, REGULATION OF P53-DEPENDENT APOPTOSIS, TRANSCRIPTIONAL REPRESSION, AND CELL-TRANSFORMATION BY PHOSPHORYLATION OF THE 55-KILODALTON E1B PROTEIN OF HUMAN ADENOVIRUS TYPE-5, Journal of virology, 71(5), 1997, pp. 3620-3627
The adenovirus type 5 55-kDa E1B protein (E1B-55kDa) cooperates with E
1A gene products to induce cell transformation. EIA proteins stimulate
DNA synthesis and cell proliferation; however, they also cause rapid
cell death by p53-dependent and p53-independent apoptosis. It is belie
ved that the role of the E1B-55kDa protein in transformation is to pro
tect against p53-dependent apoptosis by binding to and inactivating p5
3. It has been shown previously that the 55-kDa polypeptide abrogates
p53-mediated transactivation and that mutants defective in p53 binding
are unable to cooperate with E1A in transformation. We have previousl
y mapped phosphorylation sites near the carboxy terminus of the E1B-55
kDa protein at Ser-490 and Ser-491, which lie within casein kinase II
consensus sequences. Conversion of these sites to alanine residues gre
atly reduced transforming activity, and although the mutant 55-kDa pro
tein was found to interact with p53 al normal levels, it was somewhat
defective for suppression of p53 transactivation activity. We now repo
rt that a nearby residue, Thr-495, also appears to be phosphorylated.
We demonstrate directly that the wild-type 55-kDa protein is able to b
lock E1A-induced p53-dependent apoptosis, whereas cells infected by mu
tant pm490/1/5A, which contains alanine residues at all three phosphor
ylation sites, exhibited extensive DNA fragmentation and classic apopt
otic cell death. The E1B-55kDa product has been shown to exhibit intri
nsic transcriptional repression activity when localized to promoters,
such as by fusion with the GAL4 DNA-binding domain, even in the absenc
e of p53. Such repression activity was totally absent with mutant pm49
0/1/5A. These data suggested that inhibition of p53-dependent apoptosi
s may depend on the transcriptional repression function of the 55-kDa
protein, which appears to be regulated be phosphorylation at the carbo
xy terminus.