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is that this interaction inactivates Rb I function by removing it from the growth regulating
machinery of the cell, equivalent to a deletion or mutation in the RbI gene. Other
experiments suggest that the Rbi protein is involved in cell cycle regulation, probably by
regulating the progression through the GI phase of the cell cycle. Among others, the Rbi
protein is phosphorylated in a cell-cycle dependent manner: hypophosphorylated Rb I protein
predominates in G I, whereas heavily phosphorylated forms appear just prior to the G I to S
transition and persist during the S, G2 and M phase (Buchkovich et aI., 1989; DeCaprio et
aI., 1989; Chen et aI., 1989). The Rbi protein is a substrate for different kinases involved
in the cell cycle (Matsushime et aI., 1992; Ewen et aI., 1993; Kato et aI., 1993). In addition,
all three viral oncoproteins bind only to the hypophosphorylated state of the Rb I protein
(Ludlow et aI., 1989). This suggests that the hypophosphorylated form of the Rbi protein
is active in cell growth control. Moreover, hypophosphorylated instead of
hyperphosphorylated Rbi protein is able to bind the transcription factor E2F (Chellappan et
aI., 1991; Shirodkar et aI., 1992). The interaction of Rb with E2F probably blocks the
transactivation of certain positively acting growth-regulating genes such as c-myc (Thalmeier
et aI., 1989). Further study showed physical interaction between the c-myc and Rbi proteins
and microinjection of cells with Rbi and c-myc results in the inhibition of the ability of the
Rbi protein to arrest the cell cycle (Rustgi et aI., 1991; Goodrich and Lee, 1992b). All these
data together suggest that the Rbi protein functions by negatively regulating the progression
through the cell cycle by the inhibition of transcription factors involved in entry into the S
phase.
3.2 The p53 gene
The nuclear phosphoprotein p53 was originally discovered in extracts of SV40 transformed
cells (Lane and Crawford, 1979). The protein was found in a complex with SV40 large T
antigen. In the mid-1980s the corresponding gene was cloned (Matlashewski et aI., 1984;
Lamp and Crawford, 1986) and assigned to chromosome 17p13.1 (van Tuinen et aI., 1988).
Originally p53 was classified as an oncogene because cotransfection of ras and p53
genes transformed embryo fibroblasts. However, the 1'53 cDNA clones used in these
experiments were later observed to contain missense mutations. More recent experiments
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