(Received for publication, June 29, 1995; and in revised form, August 14, 1995)
From the
p53 is the most frequently mutated gene in human cancer. Naturally occurring mutations of p53 are mainly located within a region containing residues 100-300 and are predominantly of missense type, resulting in loss of the protein's DNA binding activity. Here we show that this type of mutation also represses the p53 N-terminal activation domain. The repression activity is localized in the central region of mutant p53 containing residues 101-318. Interestingly, the central region of a temperature-sensitive mutant p53N247I possesses a movable and regulable inactivation function. It represses other activities present on the same polypeptide chain without strict regard to the configuration of that polypeptide only at the nonpermissive temperature (37 °C) and not at the permissive temperature (30 °C). Furthermore, this mutant p53 region exhibits no other activity, and its function is independent of endogenous p53 status.
The tumor suppressor p53 protein is an important negative
regulator of cell proliferation(1) . Reintroduction of the wild
type p53 gene into transformed cells blocks cell proliferation (2) and causes these cells to accumulate in the late G phase of the cell cycle(3) . Loss of p53 function results
in genome instability (4, 5) and eliminates growth
arrest in response to inadequate or detrimental growth conditions at
the G
phase(6) . p53 functions as a typical
eukaryotic transcription factor; p53 binds to specific DNA sequences
termed p53-responsive elements and stimulates transcription of the
target genes(7) . Paradoxically, p53 also represses the
transcription of many viral and cellular genes that apparently do not
have p53-responsive elements(8) . In structure, p53 is
organized into three functional domains: an N-terminal region involved
in transcriptional activation, a central region mediating specific DNA
binding, and a C-terminal region responsible for oligomerization,
transcriptional repression, and nonspecific DNA
binding(7, 9) . The natural forms ofcancers almost
universally contain mutations in the p53 gene. Mostly they are single
missense point mutations located within the conserved regions 2-5
of p53 protein, resulting in loss of the protein's specific DNA
binding activity(7) .
Conditional mutations such as chimeras
between the ligand-binding domain (LBD) ()of steroid
receptors and non-receptor proteins (for a review, see (10) )
are frequently used to manipulate the activity of proteins and thereby
identify their role in various cellular processes. Nonetheless, use of
LBD chimeras has at least three disadvantages. First, steroid hormones
have receptor-independent activity(11, 12) . Second,
the hormone-binding domain possesses transactivation and
trans-repression activities(13, 14) . Third, a
receptor-free assay system may not always be available. Therefore, a
new means of creating conditional mutations would be desirable. Here we
demonstrate that the central region (CR) containing residues
101-318 of p53N247I, a temperature-sensitive mutant of the tumor
suppressor p53(15) , confers thermal regulability on chimeras
between that region and a variety of proteins. Moreover, the CR
exhibits no other activity, and its function is independent of
endogenous p53 status. These results strongly suggest that the CR can
be used as a movable regulatory cassette, a powerful tool for thermal
regulation of chimeric proteins.
To test whether the naturally occurring p53 mutations affect
the protein's transactivation function, we measured the
transcriptional activity of chimeras between p53 and GAL4 DNA-binding
domain. To eliminate complication by p53 C-terminal oligomerization (7) and repression (9, 22) from our study,
only residues 1-318 of p53 were fused to the GAL4 DNA-binding
domain. Chimera GAL4-p53(1-318) stimulated transcription in
transfected Saos-2 cells quite well at 37 °C; in contrast, chimera
GAL4-p53N247I(1-318) did not (Fig. 1A, compare lane7 to lane8). Thus, the
mutation not only caused p53 to lose its DNA binding activity (7, 15) but repressed, at least in the background of a
GAL4 chimera, the p53 N-terminal activation domain. In support, most
analogous chimeras between GAL4 and the ``hot spot'' p53
mutants(7) , including mutations at codons 143, 175, 248, 249,
and 273, failed to activate transcription. ()
Figure 1:
p53N247I
possesses temperature-sensitive transcriptional inactivation function. A, transactivation of the reporter pGE1bCAT by
GAL4-p53 chimeras at 30 °C (lanes 1-4) and at 37
°C (lanes 5-8) in Saos-2 cells by calcium
phosphate-mediated DNA transfection. Activator GAL4VP16 (17) was included as a positive control for transactivation (lanes 2 and 6). Diagrams of reporter and activator
structures are shown below the autoradiogram. Activator and relative
CAT activity (or RCA) are indicated above each track of the
autoradiogram. An autoradiogram of a typical experiment is shown. B, expression levels of GAL4-p53 derivatives. The chimeras are
the same as in A. The GAL4 derivative is indicated above each
track of the immunoblot. Molecular mass markers in kilodaltons are
shown on the left. The position of GAL4-p53 derivatives is
indicated on the right (arrowhead). Approximately 30
µg of proteins (using bovine serum albumin as a standard) from
transfected Saos-2 cells were fractionated on a 10% SDS-PAGE gel. GAL4
derivatives expressed at 30 °C (lanes2 and 3) and at 37 °C (lanes5 and 6)
were detected with an anti-GAL4(1-147)
antibody.
Since
p53N247I is temperature-sensitive as regards DNA
binding(7, 15) , we also examined the repression
activity at different temperatures. Indeed, when the temperature was
shifted from nonpermissive (37 °C) to permissive (30 °C), the
repression activity vanished (Fig. 1A, compare lane4 to lane8), indicating that this
activity was also temperature-sensitive. Control experiments
demonstrated that the two GAL4 chimeras were expressed at similar
levels (Fig. 1B) and were localized in the
nucleus regardless of the assay temperature.
Next, we defined the region mediating the p53N247I repression activity. As shown in Fig. 2, the region containing residues 101-318 of p53N247I, but not of wild-type p53, conferred thermal regulability on a heterologous activator, GAL4VP16 (23) (panelI, compare lanes7 and 8 to lanes3 and 4). Notably, however, the two chimeric proteins were expressed approximately equally at either assay temperature (data not shown), which, in conjunction with data in Fig. 1B, suggested that fusion of the mutant p53 fragment to a peptide seemed not to affect expression of the resultant chimera. Furthermore, the temperature-sensitive repression activity of the p53 fragment was not restricted to Saos-2 cells. The same phenomenon was observed in CHO (see below) and in HeLa, H1299, and HepG2 cells (Fig. 2, panels II, III, and IV). Thus, we reached three conclusions. First, the thermally regulable inactivation function of p53N247I was confined to a region encompassing residues 101-318 (called the central region or CR). Second, this function of the CR was independent of protein context (Fig. 1A and Fig. 2; also see below). Third, the endogenous p53 status had little influence on the CR's function. For instance, Saos-2 and H1299 cells expressed no p53 protein, whereas HeLa and HepG2 cells, although containing the wild-type p53 gene, differed dramatically in p53 protein level(5, 24) . However, the CR's function was similar in all four cell lines (Fig. 2).
Figure 2:
The temperature-sensitive inactivation
function of p53N247I is movable and located in the CR ranging from
residues 101 to 318 of the protein. Transactivation of the reporter
pGE1bCAT by GAL4 derivatives is shown. Procedures were as
described in Fig. 1A, except that activators are
GAL4VP16-p53 derivatives and that diagrams of their structures are
shown below the autoradiogram. Data were collected at 30 °C (lanes1-4) or at 37 °C (lanes
5-8) from Saos-2 (panel I), HeLa (panel
II), H1299 (panel III), and HepG2 (panel IV)
cells. RCA, relative CAT activity.
All studies described above used GAL4 derivatives. To test the generality of the CR's inactivation function we fused it to a totally unrelated protein, the adenovirus 13S E1a protein, which exerts complex regulatory effects on various viral and cellular promoters, such as the adenovirus E4 promoter(25) . As shown in Fig. 3, E1aCD67 (26) , an E1a derivative, stimulated E4 transcription at either temperature (compare lanes1 and 2 to lanes4 and 5) in CHO cells. Remarkably, the chimeric protein, E1aCD67-CR, stimulated transcription in a completely temperature-dependent manner (compare lanes1 and 3 to lanes4 and 6). Thus, these results parallel those obtained with GAL4VP16-CR and demonstrate that the transactivation activity of E1aCD67 can be thermally repressed by fusing E1aCD67 to the CR.
Figure 3: The CR confers thermal regulation to the adenovirus E1a protein. Transactivation of reporter pE4CAT by adenovirus E1a derivatives at 30 °C (lanes 1-3) and at 37 °C (lanes 4-6) is shown. Basal E4 promoter activity consistently was relatively higher at 30 °C than at 37 °C, which resulted in the low stimulation of E4 promoter by E1a derivatives at 30 °C (compare lanes1 and 2 to lanes4 and 5). The 13S E1a contains 289 amino acids, but a deletion mutant lacking 67 C-terminal residues (E1aCD67) can nevertheless stimulate transcription(26) . The CR was fused C-terminally to E1aCD67; E1a-CR was tested by transient expression assays performed as described in Fig. 1A, except that CHO cells were recipients for DEAE-mediated DNA transfection and that the reporter was pE4CAT. RCA, relative CAT activity.
Results obtained with GAL4VP16-CR and E1aCD67-CR are consistent with our hypothesis that the CR temperature sensitively represses other activities present on the same polypeptide chain without strict regard to the configuration of that polypeptide. Nonetheless, ideally to provide a movable and regulable inactivation function, the CR should have as few other activities as possible. Therefore, the binding of a wild-type p53 fragment containing residues 102-292 to DNA in vitro(27) seems to pose a concern about the CR as an optimal approach to creating conditional mutants. Accordingly, we determined whether the CR within a chimera possesses DNA binding activity. In this assay, p53 binding sites (or PBS hereafter) were placed immediately downstream of the reporter's TATA box. Thus, binding of p53 derivatives would block assembly of the transcription initiation complex on the promoter and therefore reduce CAT activity. p53(51-363), a derivative providing only p53 DNA binding activity (7, 9) but not GAL4VP16-CR, reduced transcription of the reporter containing PBS at 30 °C (Fig. 4, compare lane3 to lane4). The reduction was specific because transcription of a corresponding reporter lacking PBS was not affected (Fig. 4, compare lanes2 and 3 to lanes7 and 8). Thus, in the context of a chimera the CR showed no detectable affinity to PBS. Besides, the inability to bind PBS was not a peculiarity of GAL4VP16-CR, since another chimera, E1aCD67-CR, was also unable to bind PBS (Fig. 4, compare lane3 to lane5).
Figure 4:
The CR does not bind DNA at the permissive
temperature. One microgram of the illustrated reporter DNA, 1 µg of
the activator DNA, and 13 µg of the specified blocker DNA were used
for calcium phosphate-mediated DNA transfection. To prevent the
N-terminal activation activity (7) and the C-terminal
inactivation activity (9, 22) of p53 from interfering
with our study, a p53 derivative, p53(51-363), which contains
residues 51-363 of human p53, was used as a positive control for
blocking. Cells were harvested 36 h after transfection. The
absence(-) or presence (+) of activator is indicated above
each track of the autoradiogram, as are blocker and relative CAT
activity (or RCA). The reporter was
pLEP
C (lanes 1-5) or
pL
EC (lanes 6-10). Diagrams of the structure
of activator, reporters, and blockers are shown below the
autoradiogram.
We have
demonstrated that the CR possesses a movable and thermally regulable
inactivation function toward several distinct proteins. Interestingly,
among human temperature-sensitive p53 mutants(15) , p53N247I is
unique in having a CR endowed with such a function. ()How
does the CR regulate other activities present on the same polypeptide
chain? We speculate that the conformational state of the CR may play a
role, since the codon 247 mutation is located within the p53
conformation-sensitive region(7) . It is possible that within a
chimera the change of CR's conformation could propagate to the
neighboring region. This could account for loss of function of the
chimera. Alternatively, the change of CR's conformation
facilitates the protein's interaction with some other protein
which, as illustrated by the interaction between LBD and heat shock
protein 90(10) , inactivates the chimera by steric hindrance.
Further studies are required to distinguish these hypotheses.
It was puzzling to observe that the CR failed to bind DNA (Fig. 4) at the permissive temperature where the CR is supposed to assume the DNA binding conformation of p53. Perhaps this discrepancy reflects two aspects. First, p53N247I, from which the CR is derived, retains a small percentage of p53's transactivation activity at 30 °C, apparently due to its low DNA binding affinity(15) . Second, the protein concentrations (p53 versus CR) required for DNA binding may differ. If, as seems likely, the oligomerization domain within the context of p53 helps the CR to bind DNA, then the CR alone would need to be expressed to a higher concentration than that achievable in vivo to drive the protein-DNA interaction.