2. RESULTS
2.3. Pin1 influences Cdc20 distribution
2.3.3. Inhibition of Pin1 impairs localization of Cdc20 to kinetochores
Several reports indicate kinetochore localization of Cdc20 during mitosis: This localiza‐
tion is established during prophase and abolished upon anaphase onset (Kallio et al., 2002). Furthermore localization at the kinetochores has been described for all MCC components (Vigneron et al., 2004). However, it is not clear yet, whether Cdc20 localiza‐ Cdc20 signals were visibly reduced by Pin1‐immunodepletion (Fig. 25B). However, using this approach it was not possible to clarify whether Pin1 directly influences Cdc20 localization to kinetochores, as most Cdc20 was co‐depleted from the extract upon Pin1‐immunodepletion (Fig. 25C). Hence, it was decided to address the issue of a pos‐
RESULTS slightly reduced compared to mock (GL2) treatment (Fig. 26). Similar effects could be observed in mitotically arrested Hela cells treated with Pin1 inhibitor DTM (data not shown). However, also in these approaches it cannot be excluded that treatment of the cells with RNAi or inhibition of Pin1 by DTM induces side effects such as co‐depletion of Cdc20. It is furthermore possible that reduced Cdc20 levels at the kinetochores during mitosis are an indirect effect of Pin1 depletion.
To address the issue whether Pin1 directly influences Cdc20 localization, stable trans‐
genic cell lines were generated inducibly expressing wildtype Cdc20 (Cdc20WT) or a variant thereof lacking all Cdk1 phosphorylation sites (Cdc207A). In a previous experi‐
ment, this variant failed to associate with Pin1 (Fig. 22B). After a mitotic shake‐off, cells were prepared for immunofluorescence analysis using antibodies against Cdc20 and the kinetochore marker Hec1. Using this approach, localization of both wildtype Cdc20 and Cdc207A to the kinetochores was observed, indicating that Pin1 does not directly influ‐
ence Cdc20 distribution to the kinetochores (Fig. 27A).
Hence, the question was raised, whether there are other mechanisms that direct Cdc20 to the kinetochores as Pin1 didn’t seem to directly contribute to Cdc20 localization. To this end, Hela wells were transfected with plasmids coding for different Cdc20 variants shown in Fig. 27B and arrested in mitosis with nocodazole. Thereafter, immunofluores‐
cence analysis of the corresponding cells was performed and localization of Cdc20 to the kinetochores was analyzed. Cdk1‐ and Bub1‐phosphorylation deficient Cdc20 vari‐
ants still showed association with kinetochores, indicating that phosphorylation is not a prerequisite for proper Cdc20 localization. (Fig. 27C‐E). However, a C‐terminally trun‐
cated variant failed to localize to kinetochores, whereas a Cdc20 variant lacking the last two amino acids (unable to mediate association with Cdc27 or MCC component BubR1), still showed localization.
In conclusion, Pin1 does not seem to directly influence Cdc20 localization to kineto‐
chores during mitosis. The localization of Cdc20 to kinetochores is neither controlled by phosphorylation nor by cis‐trans isomerization. Instead it is regulated by an unknown mechanism, which requires the C‐terminal end of Cdc20.
RESULTS
Fig. 25: Pin1 depletion from CSF extract reduces the amount of total and kinetochore‐associated Cdc20: (A) Experimental outline of the experiment. In brief, CSF extract was immunodepleted with anti‐xPin1 antibodies or unspecific IgG (mock) and then supplemented with low amounts of sperm nuclei and rhodamine labeled tubulin. After an incubation period of 30 min at 18°C, the sperm nuclei were re‐isolated and analyzed by immunofluorescence microscopy (B) Upper panels: Representative IF pictures of re‐isolated chromatin from mock or Pin1 depleted CSF extract. Chromatin was stained using antibodies against the centromere marker xCenpA (red) and Cdc20 (green). DNA was counter‐
stained with Hoechst 33342. Scale bar is 20 µm. Lower panels: Representative IF pictures of extract samples taken after the depletion process. DNA was counterstained with Hoechst 33342. Rhod‐
amine labeled tubulin is shown in red. Scale bar is 20 µm. (C) Extract samples taken after immu‐
nodepletion were subjected to an immunoblot analysis using antibodies against xCdc20, α‐tubulin and xPin1.
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RESULTS
Fig. 26: Pin1 depletion reduces the amount of kinetochore‐bound Cdc20 in Hela cells. (A‐C) Hela cells were transfected with siRNAs directed against Pin1 mRNA (Pin1 M) or mock transfected (GL2) for 48 h. Following treatment with nocodazole for 14 h, mitotic cells were harvested by shake‐off.
Cells were then either spun on coverslips, fixed and stained for immunofluorescence microscopy or prepared for immunoblot analysis. (A) Immunofluorescence pictures of the corresponding cells stained with antibodies against Cdc20 (green) and CREST as a marker for centromeres (red). DNA was stained with Hoechst 33342. Scale bar is 10 µm. (B). Lysates of mitotic cells were analyzed by SDS‐PAGE and immunoblotting to assay efficiency of Pin1 depletion by RNAi. Mitotic cells (50 each) with kinetochoric (localized) and no detectable Cdc20 signal (not localized) were counted. Numbers are presented in a table (C).
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RESULTS
Fig. 27: A Cdc20 variant with all Cdk1 sites mutated still localizes to kinetochores during SAC sig‐
nalling, however a C‐terminally truncated variant does not. (A) Stable transgenic Hela cell lines that express Myc‐tagged wildtype Cdc20 (Cdc20WT) or a variant lacking all Cdk1 phosphorylation sites (Cdc207A) were arrested in mitosis with a double thymidine‐nocodazole block. After mitotic shake‐
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RESULTS off, cells were harvested and transferred to fresh medium containing nocodazole plus doxycyclin (+Tet) or the corresponding carrier, ethanol (‐Tet). After 6 h of incubation, cells were harvested and localization as well as expression of the transgenes were analyzed by immunofluorescence and im‐
munoblot analysis. Left panel: Immunoblot analysis of total cell lysates with anti‐Myc and anti‐α‐
tubulin antibodies. Right panel: Immunofluorescence pictures of the corresponding cells stained with antibodies against the kinetochore marker Hec1 (green) and the Myc epitope (red). DNA was stained with Hoechst 33342. (B) Schematic representation of Cdc20 variants (WT=wild type, IR= C‐
terminally truncated variant with last two amino acids Ile and Arg removed, ΔC= C‐terminally trun‐
cated variant with the last 29 amino acids deleted, 7A= Cdk1 phosphorylation‐site deficient, BPM=
Bub1 phosphorylation‐site deficient). The red circles indicate the positions of mutations introduced to destroy Cdk1 or Bub1 phosphorylation sites. (C‐E) Hela cells were transfected with plasmids cod‐
ing for different N‐terminally Flag tagged Cdc20 variants (shown in B), arrested in mitosis with no‐
codazole and harvested by mitotic shake‐off. Thereafter, cells were subjected to immunoblot and immunofluorescence analysis (C) Representative IF pictures of mitotic cells stained with antibodies against the Flag epitope (green) and CREST as a marker for centromeres (red). DNA was counter‐
stained with Hoechst 33342. Scale bar is 10 µm. (D) Immunoblot analysis with anti‐Flag and anti‐α‐
tubulin antibodies (E) Mitotic cells (30 each) with kinetochoric‐associated (localized) or no detect‐
able Flag signal (not localized) were counted. Numbers are presented in a table.