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Molecular and Cellular Biology, January 2001, p. 281-288, Vol. 21, No. 1
0270-7306/01/$04.00+0 DOI: 10.1128/MCB.21.1.281-288.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
DNA Damage-Dependent Nuclear Dynamics of the
Mre11 Complex
Olga K.
Mirzoeva and
John H. J.
Petrini*
Laboratory of Genetics, University of
Wisconsin Medical School, Madison, Wisconsin 53706
Received 20 July 2000/Returned for modification 28 August
2000/Accepted 10 October 2000
 |
ABSTRACT |
The Mre11 complex has been implicated in diverse aspects of the
cellular response to DNA damage. We used in situ fractionation of human
fibroblasts to carry out cytologic analysis of Mre11 complex proteins
in the double-strand break (DSB) response. In situ fractionation
removes most nucleoplasmic protein, permitting immunofluorescent
localization of proteins that become more avidly bound to nuclear
structures after induction of DNA damage. We found that a fraction of
the Mre11 complex was bound to promyelocyte leukemia protein bodies in
undamaged cells. Within 10 min after gamma irradiation, nuclear
retention of the Mre11 complex in small granular foci was observed and
persisted until 2 h postirradiation. In light of the previous
demonstration that the Mre11 complex associated with ionizing radiation
(IR)-induced DSBs, we infer that the protein retained under these
conditions was associated with DNA damage. We also observed increased
retention of Rad51 following IR treatment, although IR induced Rad51
foci were distinct from Mre11 foci. The ATM kinase, which
phosphorylates Nbs1 during activation of the S-phase checkpoint, was
not required for the Mre11 complex to associate with DNA damage. These
data suggest that the functions of the Mre11 complex in the DSB
response are implicitly dependent upon its ability to detect DNA damage.
 |
INTRODUCTION |
The human Mre11 complex, consisting
of Mre11, Rad50, and Nbs1, functions in diverse aspects of the cellular
response to DNA damage (43). Previous cytologic analyses
have provided evidence that this complex associates with damaged DNA.
This aspect of Mre11 complex function is diminished in human Mre11
complex mutants that exhibit S-phase checkpoint deficiency
(44). We have therefore hypothesized that the complex
influences the S-phase checkpoint either because it is itself a sensor
of DNA damage or because it functions in close proximity to a sensor
(8, 48).
In previous studies, we observed that the formation of ionizing
radiation (IR)-induced foci (IRIF) of Mre11 complex proteins was
strictly dependent upon the prior induction of double-strand breaks
(DSBs). IRIF formation and IRIF multiplicity were influenced by the
number of DSBs as well as by mutations affecting the DSB response. On
that basis, the cytology of the Mre11 complex was inferred to reflect
its role in the response to DSBs (35). Indeed, Mre11
complex IRIF formation has been used as an index of the DNA damage
response in a variety of contexts (8, 12, 17, 19, 24, 33, 48, 49,
53, 54).
The interpretation that DSB-induced formation of Mre11 complex IRIF
reflected an interaction with DNA damage was supported by the technique
of partial-volume irradiation. This methodology permits the induction
of DSBs in discrete subnuclear volumes and was used to demonstrate that
the complex associates with DSBs while DSB repair is ongoing
(39). In contrast, under the conditions previously
employed, IRIF were detectable only at time points when the vast
majority of DSB repair was complete. Thus, we have proposed that these
structures represent Mre11 complex accumulation at irreparable or
slowly repaired lesions (35). As such, IRIF observed at
later time points provide indirect information regarding the DSB
response and likely do not occur at sites of DNA damage or ongoing DSB repair.
Having used partial-volume irradiation to establish that Mre11 complex
proteins bind damaged DNA during the time course of DSB repair, a
method of in situ fractionation was adapted to analyze damaged cells at
earlier time points. We reasoned that after IR, only a subset of the
total Mre11 complex would be associated with DNA damage and the
remaining nucleoplasmic pool would obscure the damage-associated
protein from immunofluorescent detection.
We found that after detergent extraction, relocalization of Mre11
complex proteins was readily observed as early as 10 min post IR. The
kinetics of IR-induced Mre11 complex relocalization in extracted cells
were similar to those observed in partial-volume irradiation
(39). Thus, we interpret this relocalization to reflect
association of the complex with DNA damage. Surprisingly, Ku70, a
protein suggested to bind DNA ends from in vitro analyses (18,
50), did not relocalize in response to IR, nor did it colocalize
with the Mre11 complex. The ATM kinase phosphorylates Nbs1 in response
to DNA damage, and this event is required for activation of the S-phase
checkpoint (17, 32, 51, 53). In this report, we show that
ATM does not influence the complex's ability to relocalize in response
to DSB induction. This result suggests that ATM phosphorylation of Nbs1
during S-phase checkpoint activation occurs after the Mre11 complex has
associated with damaged DNA.
 |
MATERIALS AND METHODS |
Cell lines.
Human 37Lu and IMR90 primary diploid fibroblasts
were obtained from the American Type Culture Collection and were used
at passages 9 to 15 and 12 to 17, respectively. 180BR primary
fibroblasts were obtained from C. Arlett (MRC Cell Mutation Unit,
Brighton, United Kingdom) and were used at passages 14 to 20. Ataxia-telangiectasia (A-T) primary fibroblasts (AT3BI) were obtained
from J. Murnane (University of California San Francisco) and were used
at passages 16 to 20. All cells were grown at 37°C in 5%
CO2 in Dulbecco modified Eagle medium with 10% fetal calf
serum (FCS) and were negative in monthly mycoplasma tests.
Antibodies.
All of the primary immunologic reagents used
were directed against human proteins. Affinity purification of Mre11
and Rad50 rabbit antisera were previously described (11).
Nbs1 rabbit antiserum and Mre11 monoclonal antibody (MAb) 8F3 were also
previously described (8), as was Nbs1 monoclonal antibody
(48). Mouse anti-PML (promyelocytic leukemia protein) MAb
was from Santa Cruz. Rabbit Rad51 antiserum was a kind gift from A. Shinohara (Osaka University, Osaka, Japan), mouse anti-Ku70 MAb N3H10
and anti-Ku86 MAb N9C1 were obtained from R. Burgess, University of
Wisconsin-Madison. All secondary antibodies were from Jackson
Immunoresearch Laboratories.
Immunofluorescence and in situ cell fractionation.
Cells
were seeded onto 18-mm glass coverslips. At 48 h later, cells were
gamma irradiated at 12 Gy in a Mark I 137Cs source at 2.5 Gy/min or mock irradiated. After various recovery times, the cells were
processed for immunofluorescent staining. Before fixation, the in situ
cell fractionation procedure was performed as previously described
(40). Briefly, the coverslips were washed in
phosphate-buffered saline (PBS), incubated in cytoskeleton buffer [10
mM piperazine-N,N'-bis(2-ethanesulfonic acid)
(PIPES; pH 6.8), 100 mM NaCl, 300 mM sucrose, 3 mM MgCl2, 1 mM EGTA, 0.5% Triton X-100] for 5 min on ice, followed by incubation
in cytoskeleton stripping buffer (10 mM Tris-HCl [pH 7.4], 10 mM
NaCl, 3 mM MgCl2, 1% [vol/vol] Tween 40, 0.5%
[vol/vol] sodium deoxycholate) for 5 min on ice. After several washes
with ice-cold PBS, the cells were fixed in modified Streck tissue
fixative for 30 min and permeabilized in 0.5% Triton X-100 solution
for 15 min at room temperature as previously described
(29). Cells were blocked with 10% FCS in PBS and
incubated with primary antibody for 1 h and with secondary antibody for 30 min at room temperature. All antibodies were diluted in
5% FCS-PBS. Cells were then washed, counterstained with
4',6'-diamidino-2-phenylindole (DAPI), and mounted as previously
described (35). Primary antibody dilutions were as
follows: Mre11, Nbs1, and Rad50 antisera, 1:300, 1:1,000, and 1:200
respectively; mouse anti-Mre11 and anti-Nbs1, both 1:200; mouse
anti-PML, 1:400; Rad51 antiserum, 1:200; mouse anti-Ku70 and anti-Ku86,
both 1:400. All secondary antibodies were used at 1:150, except goat
anti-rabbit immunoglobulin G-Texas Red, which was used at 1:1,000.
Controls for double labeling were performed in which one of the primary
antibodies was omitted or replaced with preimmune serum.
Most of the results presented here were obtained by immunostaining with
Mre11 antiserum. However, similar results were obtained with Mre11 MAb,
both Nbs1 MAb and polyclonal antibody, and Rad50 antiserum.
Images were captured by using a charge-coupled device camera (Princeton
Instruments, Trenton, N.J.), and grayscale images
were processed by
using IP Labs software (Signal Analytics Corporation,
Vienna, Va.) and
Photoshop 5.5 (Adobe, San Jose, Calif.).
 |
RESULTS |
We adapted an in situ fractionation procedure to examine
functional relationships between the Mre11 complex and other proteins involved in the response to DSBs. These experiments provide novel insight because they compare the intranuclear dispositions of DSB
response proteins while DNA damage is present and DSB repair is ongoing.
Three distinct localization patterns of the Mre11 complex in
response to DNA damage.
We used indirect immunofluorescence to
observe the Mre11 complex following in situ fractionation of primary
human fibroblasts. 37Lu and IMR90 normal human diploid fibroblasts were
gamma irradiated and detergent extracted prior to fixation. Subsequent
indirect immunofluorescence analysis with Mre11 and Nbs1 antisera
revealed three distinct localization patterns for the proteins of
the Mre11 complex.
The majority (60 to 90%) of unirradiated cells contained large
aggregates of Mre11, Rad50, and Nbs1. These structures also
stain with
antibody to PML, in agreement with previous studies
(
34)
(Fig.
1 and
2A and
C); Nbs1 and Rad50 data not shown).
In
contrast, from 20 to 120 min after IR, a punctate pattern of
evenly
distributed IRIF was observed (pattern II, gray bars in
Fig.
1). The
increase in pattern II IRIF was associated with a
concomitant decrease
in Mre11 staining of PML bodies (i.e., the
type I pattern) (Fig.
1 and
2C). Increased numbers of cells exhibiting
pattern II were detectable
as early as 10 min after IR and by
20 min post IR, 50 to 80% of cells
exhibited this pattern, with
a maximum at 2 h post IR (70 to 95%
of cells). At 2 h, the type
II pattern began to change and Mre11
foci often looked larger
and more sparse than at 20 min post IR (Fig.
2A). Based on previous
assessments of DSB repair kinetics in IMR90 and
37Lu (
35,
39),
the formation of pattern II coincides
temporally with DSB repair.
Mre11 complex association with PML bodies
decreased substantially
following DNA damage, and by 2 h post IR,
most PML bodies did
not stain with Mre11 antiserum (Fig.
2C). We did
not detect any
changes in multiplicity or size of PML bodies following
response
to IR.

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FIG. 1.
Kinetics of Mre11 complex relocalization in response to
DNA damage. 37Lu, IMR90 (normal diploid fibroblasts), 180BR (DNA ligase
IV-deficient fibroblasts), or AT3BI (ATM-deficient fibroblasts) cells
were either irradiated at 12 Gy or mock irradiated and processed for
immunofluorescent staining with Mre11 antiserum at the indicated time
points following IR. The percentages of cells displaying patterns I
(white bars), II (gray bars), and III (black bars) were calculated
after scoring at least 200 nuclei for each time point. Data are the
mean ± the standard deviation of three to six independent
experiments.
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FIG. 2.
Mre11 complex relocalization in normal and mutant cells.
Cells were irradiated or mock treated and processed for single
immunofluorescent staining with Mre11 antiserum (A and B) or for double
staining with Mre11 antiserum (green) and PML (red) (C) or Ku70 (red)
(D) MAb at the indicated time points after IR in normal human 37Lu
fibroblasts (A, C, and D) or AT3BI A-T fibroblasts (B). Grayscale
images were pseudocolored using IP Labs software and superimposed in
Adobe Photoshop 5.5.
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|
At 8 h post IR, the type II staining pattern decreased and the
pattern III of Mre11 localization appeared (black bars in Fig.
1). The
large, irregularly shaped foci that define this time point
were
identical to IRIF (
35). IRIF appear to result from
aggregation
of type II pattern foci or by accretion of Mre11 complex
proteins
to a subset of the type II IRIF present at early time points
(

2
h). IRIF observed at 8 h did not coincide with PML bodies,
and
at 24 h post IR, Mre11 complex proteins resumed their
association
with PML bodies (Fig.
2C).
In summary, we observed a cyclical progression of Mre11 complex
localization that began with the type I PML-associated pattern
in
unirradiated cells, followed by the formation of type II IRIF
within 10 to 20 min after IR treatment (Fig.
1 and
2A and C).
By 8 h, these
structures appeared to coalesce into type III IRIF,
which ultimately
disappear so that at 24 h post IR, the Mre11
complex was again
seen associated with PML
bodies.
The Mre11 complex is cytologically distinct from Ku70, Ku86, and
Rad51.
Genetic analysis of Saccharomyces cerevisiae has
clearly demonstrated that the Mre11 complex functions in
recombinational DNA repair, affecting both homologous recombination and
nonhomologous end joining (NHEJ) (23, 43). In contrast,
the Ku heterodimer appears to function specifically in NHEJ
(9), whereas Rad51 is uniquely required for DSB repair by
homologous recombination (27). We asked whether these
disparate functions would manifest as distinct cytologic behaviors in
IR-treated cells.
In unirradiated cells, in situ cell fractionation revealed a punctate
staining of Ku70 and Ku86 evenly distributed throughout
the
nucleus. Ku70 (Fig.
2D) and Ku86 (data not shown) remained
in
essentially the same dispersed punctate pattern following irradiation,
in contrast to Mre11 complex proteins. Similarly, the Ku complex
in
nonextracted cells was uniformly distributed in the nucleus
irrespective of prior IR treatment (data not
shown).
Under the conditions used, IR-induced alterations in the disposition of
the Ku complex were not detected. Nevertheless, the
role of the Mre11
complex in NHEJ suggested by analysis of
S. cerevisiae mre11
mutants prompted us to assess whether IR-dependent
Mre11 complex foci
would colocalize with Ku70 or Ku86. Irradiated
cells were fractionated,
fixed, and doubly stained with Ku70 MAb
and Mre11 antiserum. Ku70 and
Mre11 did not colocalize at PML
bodies in unirradiated cells or at
other nuclear structures at
any time points following irradiation (Fig.
2D; Ku86 data not
shown).
We previously showed that Rad51 and Mre11 IRIF did not colocalize nor
were they coincident within the nucleus of a given cell
at 8 h
post IR (
35). This result was somewhat surprising in
light
of the Mre11 complex's role in homologous recombination
(
6,
23). We asked whether Mre11 and Rad51 foci would colocalize
in
detergent-extracted cells at time points coinciding with ongoing
DSB
repair. Rad51 staining was not altered at 20 min post IR.
However, at 2 and 8 h after IR, about 20% of cells contained Rad51
nuclear
foci, as shown previously in nonextracted cells (
22)
(Fig.
3 and Table
1). Whereas the majority of cells (70 to
95%)
showed relocalization of Mre11 in type II IRIF between 20 and
120 min post IR, the reorganization of Rad51 into foci occurred
only in a
subset (about 20%) of these cells over this time course
(Table
1). We
did not observe significant colocalization of Mre11
and Rad51 IRIF
(Fig.
3).

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FIG. 3.
Cytologic behavior of Mre11 and Rad51 proteins. 37Lu
cells were irradiated and processed for double immunofluorescent
staining with Rad51 antiserum and Mre11 MAb 8F3 2 h post IR. Note
that although both cell types display pattern II of Mre11 localization,
only one of them is positive for Rad51 foci. An enlarged fragment
of this cell is shown as the insert to demonstrate the lack of
colocalization of Mre11 and Rad51 foci.
|
|
When comparing the Rad51 and Mre11 complex responses, it is important
to note that Rad51 levels are very low in G
1 cells and
are
not induced by IR treatment (
4,
15,
52). In contrast,
the
levels of Mre11 and Rad50 are relatively high and constant
irrespective
of cell cycle phase or the presence of DNA damage
(
11,
35). Hence, many fewer cells in an asynchronous culture
are
competent to form Rad51 as opposed to Mre11 foci. Consistent
with
previous data obtained with nonextracted cells (
35,
39),
Mre11 and Rad51 foci did not colocalize after detergent pretreatment
of
cells (Fig.
3). Coincident formation of Mre11 and Rad51 foci
in
irradiated cells was not observed in previous studies of nonextracted
cells (
35). The reduction in background staining afforded
by
pre-extraction of nucleoplasmic protein may increase the sensitivity
of immunofluorescent detection and permit detection of Mre11 and
Rad51
foci. Nevertheless, the general lack of colocalization is
consistent
with the distinct roles of the Mre11 complex and Rad51
in the DSB
response.
Genetic determinants of cytologic behavior.
The Mre11 complex
influences diverse endpoints in the cellular DNA damage response,
including DNA recombination and cell cycle regulation (6, 23,
44). In the latter functions, the Mre11 complex and the ATM
kinase collaborate to effect cell cycle checkpoint functions in S phase
(44). To assess whether Mre11 complex
relocalization was dependent upon ATM, we undertook cytologic analysis
of cells with defects in DNA ligase IV and in the ATM kinase following detergent extraction.
The DNA ligase IV mutant cell line 180BR is profoundly DSB repair
deficient (
1,
2,
45). We previously showed that
the
half-life of IR-induced DSBs and associated relocalization
of Mre11
were markedly increased in this cell line (
35,
39).
Consistent with the observation that the formation of type II
IRIF is
concomitant with DSB repair, we found that the progression
from type II
foci into PML-associated pattern I occurred more
slowly in 180BR than
in normal cells. At 8 h postirradiation,
15 to 30% of the normal
cells returned to their unirradiated pattern
of Mre11 localization,
PML-associated pattern I. In contrast,
PML-associated pattern I was not
observed in 180BR cells at this
time point and at 24 h, a
substantial fraction of the IR-treated
180BR cells remained type III
IRIF positive (Fig.
1). Hence, the
persistence of IR-induced DSBs is
correlated with the persistence
of
IRIF.
To test whether ATM activity is required for the complex to bind DNA
damage, we examined the kinetics of DNA damage-dependent
Mre11 complex
relocalization in AT3BI primary A-T fibroblasts.
With regard to the
formation of IRIF that appear over the time
course of DNA repair
(type II), Mre11 and Nbs1 relocalization
in these A-T cells was
indistinguishable from that in the wild
type (Fig.
1 and
2A and B).
However, we did not detect IRIF-positive
AT3BI cells following
detergent extraction, whereas IRIF were
present in irradiated cultures
of the same cells without extraction
(data not shown). In previous
studies of nonextracted cells, we
found that IRIF formation was
severely reduced in simian virus
40-transformed A-T cells whereas
primary A-T cells were only minimally
affected (
35,
48).
Therefore, we conclude that IRIF did form
in AT3BI cells but were
removed by detergent extraction during
in situ fractionation. Given the
lack of an effect on the formation
of type II IRIF, ATM-mediated Nbs1
phosphorylation does not appear
to influence DNA damage recognition by
the Mre11
complex.
 |
DISCUSSION |
In situ fractionation was carried out on gamma-irradiated cells to
assess alterations of the intranuclear disposition of proteins that mediate recombinational DNA repair. The Mre11 complex and the Ku
heterodimer exhibit distinct cytologic behaviors during the cellular
response to DSBs. The data further indicate that association of the
Mre11 complex with DNA damage does not require the ATM kinase.
Early versus late IRIF.
The formation of Mre11 complex late
IRIF occurs in the normal cellular response to DSBs. Conversely, this
process is affected by mutations that alter the cellular DSB response
(8, 35, 48). These observations support the utility of
IRIF formation as an index of the cellular DSB response. An important
caveat to this interpretation is that late IRIF formation is not
readily detectable until the bulk of DSB repair is complete (4 to
8 h). By using ultrasoft X rays to induce localized DNA damage,
the Mre11 complex has been directly linked to sites of DNA damage within the time course of DSB repair (30 to 90 min) (39).
However, this methodology is limiting because it precludes analysis
prior to 30 min post IR, precludes manipulations such as cell synchrony or microinjection, and requires a synchrotron to generate ultrasoft X rays.
Based on the anticipation that Mre11 complex members and other proteins
that function in DSB detection or repair would become
more avidly
associated with chromatin in the presence of DNA damage,
we adapted a
method of detergent extraction to examine gamma-irradiated
cells.
Relocalization of the Mre11 complex occurred within 10
min of DSB
induction and persisted until approximately 2 h post
IR. The type
II pattern is also seen in unirradiated cells within
S phase,
suggesting that reorganization of the complex is induced
by naturally
occurring breaks during DNA replication (R. S. Maser
et al.,
unpublished data). Interestingly, the formation of early
(i.e., type
II) IRIF is accompanied by a decrease in Mre11 complex
staining of PML
bodies (Fig.
2C). The function(s) of PML bodies
is not clearly
established, nor is it clear that their composition
is uniform in all
cell types and under all cellular conditions.
A number of enzymes with
protein modification activities reside
in PML bodies (
30,
36). Therefore, it is conceivable that
the departure of Mre11
complex members from PML bodies is attributable
to DNA damage-induced
modification of Mre11 complex or other PML-associated
proteins.
Prior to extraction, the majority of the Mre11 complex was present in
the nucleoplasm based on analysis of extracts prepared
under conditions
identical to those employed for in situ fractionation
(data not shown).
Hence, Mre11 complex proteins that ultimately
associate with DNA damage
most likely originate in the nucleoplasmic
pool. In this regard, the
ideas that PML bodies serve as reservoirs
for Mre11 complex proteins
and that Mre11 proteins leave PML bodies
to associate with DNA damage
seem less favorable, although the
data do not rigorously exclude these
possibilities.
By 8 h, a subset of the early IRIF appeared to coalesce into
large, irregularly shaped aggregates. This observation is consistent
with the interpretation that IRIF seen in nonextracted cells at
late
time points represent irreparable or slowly repaired lesions
that
nucleate multimerization of the Mre11 complex (
35). In
this regard, it is interesting that the time course over which
chromosomal aberrations accumulate in irradiated cells corresponds
well
to the 6- to 12-h time course over which IRIF appear in nonextracted
cells (
7,
35).
In DSB repair-deficient 180BR cells, the half-life of IR-induced DSBs
is markedly increased (
2) and the transition from
type II
IRIF to the PML-associated pattern is delayed in those
cells (Fig.
1).
This validates the interpretation that type II
IRIF correspond to sites
of DNA damage and repair and suggests
that the formation of IRIF is
dependent upon the process of DSB
repair.
Independent functions of the Mre11 and Ku complexes.
The role
of the Ku complex in NHEJ is firmly established by in vitro and in vivo
analyses (3, 28, 37). The observation that Ku heterodimers
bind DNA ends in vitro and activate the DNA-dependent protein kinase
catalytic subunit strongly suggests the mechanism by which these
proteins function in intact cells to facilitate NHEJ (13, 14, 25,
47). In this context, our inability to detect IR-induced
relocalization of the Ku complex is unexpected. It is conceivable that
the presumptive Ku-DNA complex is not stable under the extraction
conditions employed. However, a substantial amount Ku was retained
after extraction, irrespective of prior irradiation. This associated
fraction may represent Ku complex functions not directly related to DNA
damage processing (10, 14, 16, 20, 21, 42).
A recent report presented evidence that hamster Mre11 and Ku
colocalized at radiation-induced foci and that the mouse Mre11
and Ku
proteins physically interacted (
19). Given that we failed
to detect relocalization of Ku complex proteins by in situ
fractionation
in human cells, it is perhaps not surprising that we
failed to
observe colocalization of Mre11 at Ku foci in these
experiments.
This may reflect a difference in the reagents used or
possibly
in the properties of rodent and human Ku complex proteins. It
is clear from genetic analyses of
S. cerevisiae that the
yeast
Mre11 and Ku complexes function in distinct pathways (
5,
6,
31,
41). Crosses between Ku-deficient mice and murine Mre11
complex mutants will be important to address the functional
interactions
between these complexes in
mammals.
Mre11 complex association with DNA damage is ATM independent.
Mutations in MRE11 and NBS1 comprise the genetic
basis for two human syndromes that bear phenotypic similarity to A-T.
The A-T-like disorder (A-TLD) and Nijmegen breakage syndrome (NBS) are
caused by mutations in the MRE11 and NBS1 genes,
respectively (8, 48). The mechanistic basis for the
phenotypic similarities of A-T and NBS and A-TLD cells is unclear. It
has been established that ATM phosphorylates Nbs1 (17, 32, 51,
53) and that this event is necessary for activation of the
S-phase checkpoint (32, 53). In NBS and A-TLD cells, the
formation of Mre11, as well as Nbs1 IRIF, is virtually abrogated
(8, 48). Similarly, we were unable to detect the formation
of early (type II) IRIF of either Mre11 or Nbs1 in these cells (data
not shown). In NBS and A-TLD cells, the Mre11 complex is predominantly
cytoplasmic; thus, the failure to form foci when breaks are present may
be due in part to the complex's aberrant subcellular localization (44).
In contrast, the subcellular localization of the Mre11 complex is
normal in A-T cells (R. S. Maser, unpublished data). The
formation
of Mre11 (Fig.
3) and Nbs1 (data not shown) type II
IRIF in that
context was indistinguishable from that of wild-type
cells. This result
argues that ATM-mediated Nbs1 phosphorylation
is not required for the
Mre11 complex to associate with DNA damage.
These data support the
hypothesis that the influence of the Mre11
complex on S-phase
checkpoint activation is dependent upon DNA
damage detection, as well
as Nbs1 phosphorylation. This raises
the possibility that ATM
phosphorylates Nbs1 that is bound to
DSBs, as opposed to nucleoplasmic
Nbs1. Finally, because DSB repair
is not grossly affected in A-T cells,
phosphorylation of Nbs1
does not appear relevant to the DSB repair
functions of the complex
(
26,
38,
46).
Conclusion.
We used in situ fractionation and
immunofluorescence to analyze functional relationships among proteins
with demonstrated roles in the cellular response to DNA damage. The
primary advantage of this approach is that it facilitates cytologic
examination of DSB repair using conventional irradiation while DSBs are
present. Beyond providing further support for the hypothesis that the
Mre11 complex detects DNA damage, the data define distinct functional roles in this regard for the Mre11 complex and the Ku complex. Finally,
the data demonstrate that the ability of the Mre11 complex to bind DNA
damage does not depend upon the ATM kinase.
 |
ACKNOWLEDGMENTS |
We are grateful to the members of the lab for insights throughout
the course of this study and to Mark Kaplan and Mary Ellen Perry for
comments and suggestions on the manuscript.
This work was supported by the Milwaukee Foundation, the National
Institutes of Health (GM56888 and GM59413), and the Department of
Energy (ER62859).
 |
FOOTNOTES |
*
Corresponding author. Mailing address: Laboratory of
Genetics, University of Wisconsin Medical School, 445 Henry Mall,
Madison, WI 53706. Phone: (608) 265-6043. Fax: (608) 262-2976. E-mail: jpetrini{at}facstaff.wisc.edu.
Report 3564 from the University of Wisconsin-Madison Laboratory of Genetics.
 |
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Molecular and Cellular Biology, January 2001, p. 281-288, Vol. 21, No. 1
0270-7306/01/$04.00+0 DOI: 10.1128/MCB.21.1.281-288.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
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