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First published online September 17, 2008
doi: 10.1242/10.1242/jcs.034702


Journal of Cell Science 121, 3224-3232 (2008)
Published by The Company of Biologists 2008
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Cyclin E-dependent localization of MCM5 regulates centrosome duplication

Rebecca L. Ferguson and James L. Maller*

Howard Hughes Medical Institute and Program in Molecular Biology, University of Colorado School of Medicine, Aurora, CO 80045, USA


Figure 1
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Fig. 1. MCM5 specifically interacts with the wild-type CLS of cyclin E. (A) Endogenous cyclin E co-immunoprecipitates MCM5. Lysates from asynchronous HeLa S3 cells were subjected to immunoprecipitation, as described in the Materials and Methods, with anti-cyclin E antibody or control IgG, as indicated, and immunoprecipitates were western blotted with anti-MCM5 and -cyclin E antibodies. (B) Cyclin E directly interacts with full-length MCM5. MCM5 was radiolabeled with [35S]methionine in an in vitro transcription-translation system as described in the Materials and Methods. The radiolabeled protein was incubated with glutathione-agarose beads bound to GST or GST–cyclin E. Beads were washed, eluted with SDS-PAGE sample buffer and electrophoresed on 10% gels. Left: Coomassie-stained gel of 25% of GST and GST–cyclin E. Right: an autoradiograph of MCM5 is shown. (C) Mutation of the cyclin E CLS disrupts MCM5–cyclin E interaction. Flp-In T-Rex CHO cells were induced to express the indicated Myc-tagged cyclin E constructs as described in the Materials and Methods. Lysates were prepared and anti-Myc immunoprecipitates were electrophoresed on 10% SDS gels, followed by western blotting for MCM5 (top panel), Myc (middle panel) and Cdk2 (lower panel).

 

Figure 2
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Fig. 2. The cyclin E-interaction domain within MCM5. (A) HA-tagged MCM5 cyclin E-interaction fragments. The indicated HA-tagged MCM5 fragments were radiolabeled with [35S]methionine in an in vitro transcription-translation system and subjected to pulldown analysis as in Fig. 1B with GST or GST–cyclin E. Right: an autoradiograph is shown. Left: a Coomassie-stained gel of 25% of GST and GST cyclin E. (B) GFP-tagged MCM5 cyclin E-interaction fragments. The indicated GFP-tagged fragments were radiolabeled and analyzed by GST pulldown analysis as in A. Right: autoradiograph of pulldown. Left: Coomassie-stained gel of 25% of GST and GST proteins.

 

Figure 3
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Fig. 3. Evolutionary conservation of the MCM5 domain that interacts with cyclin E. The region from human MCM5 that is sufficient for binding cyclin E was aligned with MCM5 proteins from the indicated species. Red residues are identical, blue residues are highly similar and green residues are moderately similar.

 

Figure 4
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Fig. 4. Localization of MCM5 on centrosomes. (A) Endogenous MCM5. Asynchronous CHO-K1 or HeLa S3 cells were methanol-fixed and stained with antibody to {gamma}-tubulin and MCM5, as indicated. Arrowheads mark the centrosome. Inset: enlarged image of the centrosomal area. (B) Transfection of HA-tagged MCM5. CHO-K1 cells were transiently transfected with the indicated MCM5 constructs as described in the Materials and Methods. Cells were methanol-fixed and stained with antibodies to {gamma}-tubulin and HA, as indicated. Arrowheads mark the centrosome. Inset: enlarged image of the centrosomal area. (C) Transfection of GFP-tagged MCM5 fragments. The indicated constructs were expressed in CHO-K1 cells and stained with antibody to {gamma}-tubulin. GFP was visualized by excitation of the expressed GFP tag. Arrowheads mark the centrosome. Inset: enlarged image of the centrosomal area. Scale bars: 5 µm.

 

Figure 5
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Fig. 5. Effect of PACT–cyclin E expression. (A) Localization of PACT-domain-fused cyclin E. The indicated 6Myc-tagged PACT–cyclin E fusion proteins were expressed by transient transfection in CHO-K1 cells, methanol-fixed, and analyzed by immunofluorescence for {gamma}-tubulin and Myc–cyclin E. Arrowheads mark the centrosome. Inset: enlarged image of merged centrosomal area. Scale bars: 5 µm. (B) MCM5 centrosomal localization relies on an interaction with cyclin E. Endogenous MCM5 centrosomal localization was analyzed as in Fig. 4A in CHO-K1 cells co-expressing either GFP or GFP-CLS along with the indicated Myc-tagged PACT-fused cyclin E fusion constructs. Over 500 cells were analyzed for each condition in each experiment. Error bars indicate mean ± s.e.m. (n 3).

 

Figure 6
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Fig. 6. Expression of MCM5 but not MCM2 inhibits centrosome duplication. (A) Effect of MCM5 on centrosome number. HU-arrested CHO-K1 cells were transiently transfected with full-length HA-tagged MCM5. The number of {gamma}-tubulin-staining foci (centrosomes) was monitored by immunofluorescence in HA-expressing (white bars) and non-expressing (control) cells (black bars) 24 hours after transfection. (B) Statistical analysis of the effect of MCM5 on centrosome duplication. Several experiments similar to the one in A were performed with the indicated constructs. The bar indicates mean ± s.e.m. (n 3). Over 150 cells were counted for each condition in each experiment.

 

Figure 7
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Fig. 7. Expression of MCM5 does not displace endogenous cyclin E from centrosomes. CHO-K1 cells transiently transfected with full-length HA-tagged MCM5 were methanol-fixed after 24 hours and analyzed by immunofluorescence for the localization of endogenous cyclin E. Arrowheads mark the centrosome. Inset: enlarged image of the centrosomal area. Scale bar: 5 µm.

 

Figure 8
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Fig. 8. Model for positive and negative regulatory roles of cyclin E–Cdk2 and MCM5 during DNA and centrosome replication. A positive role for cyclin E–Cdk2 is indicated by the arrowhead, e.g. phosphorylation of Cdc45 to initiate origin firing during DNA replication or of nucleophosmin B to initiate centrosome duplication. The inhibitory arm denotes the role of cyclin E–Cdk2 in preventing re-replication of DNA or centrosomes in the same cycle (e.g. by preventing re-loading of MCM proteins onto origins of replication on DNA or recruiting MCM5 to centrosomes).

 

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