Mark Jackman

Active cyclin B1-Cdk1 first appears on centrosomes in prophase

Cyclin B1–Cdk1 is the key initiator of mitosis, but when and where activation occurs has not been precisely determined in mammalian cells. Activation may occur in the nucleus or cytoplasm, as just before nuclear envelope breakdown, Polo-like kinase1 (Plk1) is proposed to phosphorylate cyclin B1 in its nuclear export sequence (NES), to trigger rapid nuclear import. We raised phospho-specific antibodies against cyclin B1 that primarily recognise the active form of the complex. We show that cyclin B1 is initially phosphorylated on centrosomes in prophase and that Plk1 phosphorylates cyclin B1, but not in the NES. Furthermore, phosphorylation by Plk1 does not cause cyclin B1 to move into the nucleus. We conclude that cyclin B1–Cdk1 is first activated in the cytoplasm and that centrosomes may function as sites of integration for the proteins that trigger mitosis.

MPF localization is controlled by nuclear export.

A.Hagting and C.Karlsson contributed equally to this work

Cyclin/Cdk-dependent initiation of DNA replication in a human cell-free system.

We describe a cell-free system from HeLa cells that initiates DNA replication under cell cycle control. G1 but not G2 phase nuclei initiate replication when coincubated with S phase nuclei in cytosolic extracts from S phase but not from G1 or G2 phase HeLa cells. S phase nuclei or an S phase nuclear extract are required for the initiation of semiconservative DNA replication in G1 nuclei but not for elongation in S phase nuclei. S phase nuclear extract could be replaced by recombinant human cyclins A and E complexed to Cdk2 but not by Cdk2 alone or by human cyclin B1 complexed to Cdc2. In S phase cytosol, cyclins A/Cdk2 and E/Cdk2 triggered initiation synergistically.

Cdc2 Kinase Directly Phosphorylates the cis-Golgi Matrix Protein GM130 and Is Required for Golgi Fragmentation in Mitosis

Mitotic fragmentation of the Golgi apparatus can be largely explained by disruption of the interaction between GM130 and the vesicle-docking protein p115. Here we identify a single serine (Ser-25) in GM130 as the key phosphorylated target and Cdc2 as the responsible kinase. MEK1, a component of the MAP kinase signaling pathway recently implicated in mitotic Golgi fragmentation, was not required for GM130 phosphorylation or mitotic fragmentation either in vitro or in vivo. We propose that Cdc2 is directly involved in mitotic Golgi fragmentation and that signaling via MEK1 is not required for this process.

Human cyclins B1 and B2 are localized to strikingly different structures: B1 to microtubules, B2 primarily to the Golgi apparatus.

We have raised and characterized antibodies specific for human cyclin B2 and have compared the properties of cyclins B1 and B2 in human tissue culture cells. Cyclin B1 and B2 levels are very low in G1 phase, increase in S and G2 phases and peak at mitosis. Both B‐type cyclins associate with p34cdc2; their associated kinase activities appear when cells enter mitosis and disappear as the cyclins are destroyed in anaphase. However, human cyclins B1 and B2 differ dramatically in their subcellular localization. Cyclin B1 co‐localizes with microtubules, whereas cyclin B2 is primarily associated with the Golgi region. In contrast to cyclin B1, cyclin B2 does not relocate to the nucleus at prophase, but becomes uniformly distributed throughout the cell. The different subcellular locations of human cyclins B1 and B2 implicate them in the reorganization of different aspects of the cellular architecture at mitosis and indicate that different mitotic cyclin‐cyclin‐dependent kinase complexes may have distinct roles in the cell cycle.

Translocation of cyclin B1 to the nucleus at prophase requires a phosphorylation-dependent nuclear import signal

BACKGROUND At M phase, cyclin B1 is phosphorylated in the cytoplasmic retention sequence (CRS), which is required for nuclear export. During interphase, cyclin B1 shuttles between the nucleus and the cytoplasm because constitutive nuclear import is counteracted by rapid nuclear export. In M phase, cyclin B moves rapidly into the nucleus coincident with its phosphorylation, an overall movement that might be caused simply by a decrease in its nuclear export. However, the questions of whether CRS phosphorylation is required for cyclin B1 translocation in mitosis and whether a reduction in nuclear export is sufficient to explain its rapid relocalisation have not been addressed. RESULTS We have used two forms of green fluorescent protein to analyse simultaneously the translocation of wild-type cyclin B1 and a phosphorylation mutant of cyclin B1 in mitosis, and correlated this with an in vitro nuclear import assay. We show that cyclin B1 rapidly translocates into the nucleus approximately 10 minutes before breakdown of the nuclear envelope, and that this movement requires the CRS phosphorylation sites. A cyclin B1 mutant that cannot be phosphorylated enters the nucleus after the wild-type protein. Phosphorylation of the CRS creates a nuclear import signal that enhances cyclin B1 import in vitro and in vivo, in a manner distinct from the previously described import of cyclin B1 mediated by importin beta. CONCLUSIONS We show that phosphorylation of human cyclin B1 is required for its rapid translocation to the nucleus towards the end of prophase. Phosphorylation enhances cyclin B1 nuclear import by creating a nuclear import signal. The phosphorylation of the CRS is therefore a critical step in the control of mitosis.

Assays for CDK activity and DNA replication in the cell cycle.

This unit describes two assays for different stages of the cell cycle in tissue culture cells. One is a biochemical measurement that assays the protein kinase activity of different cyclin‐dependent kinase complexes that are present in late G1 phase, S phase, G2 phase, or mitosis. The other assay uses immunofluorescence to detect DNA replication by the incorporation of nucleotide analogs into DNA. These assays are useful in analyzing the stage and degree of synchrony of the cell cycle and changes in the basic cell cycle machinery.