Reconstitution of cyclin-dependent cdc2 and cdk2 kinase activities in vitro

Cyclin B1/Cdk1 coordinates mitochondrial respiration for cell-cycle G2/M progression

A substantial amount of mitochondrial energy is required for cell-cycle progression. The mechanisms underlying the coordination of the mitochondrial respiration with cell-cycle progression, especially the G2/M transition, remain to be elucidated. Here, we show that a fraction of cyclin B1/Cdk1 proteins localizes to the matrix of mitochondria and phosphorylates a cluster of mitochondrial proteins, including the complex I (CI) subunits in the respiratory chain. Cyclin B1/Cdk1-mediated CI phosphorylation enhances CI activity, whereas deficiency of such phosphorylation in each of the relevant CI subunits results in impairment of CI function. Mitochondria-targeted cyclin B1/Cdk1 increases mitochondrial respiration with enhanced oxygen consumption and ATP generation, which provides cells with efficient bioenergy for G2/M transition and shortens overall cell-cycle time. Thus, cyclin B1/Cdk1-mediated phosphorylation of mitochondrial substrates allows cells to sense and respond to increased energy demand for G2/M transition and, subsequently, to upregulate mitochondrial respiration for successful cell-cycle progression.

Are histones, tubulin, and actin derived from a common ancestral protein?

Histones and the cytoskeletal components tubulin and actin all act as thermal ratchets, using the energy present in Brownian motion to do work. All three also bind to nucleotides. Here we suggest that histones, tubulin, and actin derive from a common ancestral protein. There is some sequence similarity between histone 2A and the bacterial tubulin homologue FtsZ. Histones and actin also share some sequence similarity in the nucleotides and at phosphate-binding sites. Thus, actin and tubulin may also be related, although this is not obvious from sequence analysis. Indeed, actin and tubulin are closely functionally related and cooperate in many cellular processes. Interestingly, recent advances in nanotechnology suggest that thermal ratchets may be able to impart lifelike properties; thus, the evolution of the ancestral histone, tubulin, and actin thermal ratchet may have been crucial in the development of complexity in living organisms.

Adenosine deaminase acting on RNA 1 accelerates cell cycle through increased translation and activity of cyclin-dependent kinase 2

Adenosine-to-inosine RNA editing has been recently implicated in the pathogenesis of inflammation through the upregulation of the editase adenosine deaminase acting on RNA 1 (ADAR1). Because cell proliferation is a key feature of the inflammatory process, the present study tested the hypothesis that overexpression of ADAR1 accelerates cell cycle. To that end, human embryonic kidney 293 cells were transiently transfected with ADAR1 or vector, and cell cycle was evaluated by fluorescence-activated cell sorter. Overexpression of wild-type ADAR1 decreased the proportion of G0-G1 cells (-19%, P<0.01, n=3), increased the percentage of S phase cells (+19%, P<0.01, n=3), and did not change the ratio of cells residing in the G2-M phase (n=3). This finding was supported by three observations. First, there was a parallel production in ADAR1-transfected cells of cyclin-dependent kinase (Cdk) 2 and cyclin A, a pivotal protein complex upregulated at the G1-S phase checkpoint, and of [p]-Histone H1, a marker of Cdk2 activity (+102%, P<0.01, n=3). Second, ADAR1-transfected cells displayed higher activity of the proliferation marker, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide. Third, using anti-ADAR1 antibody, direct binding of ADAR1 to Cdk2 messenger RNA was demonstrated in ADAR1-transfected cells by protein-RNA cross-linking and immunoprecipitation (+974%, P<0.01, n=3). Finally, causal relationships between ADAR1 and Cdk2 were confirmed by a study with the Cdk2 inhibitor, kenpaullone, which prevented the ADAR1-induced shift from the G0-G1 to the S phase. Taken together, these data show that ADAR1 increases cell cycle by shifting cells from the G0-G1 to the S phase through the upregulation of Cdk2.

Inhibitors of Leishmania mexicana CRK3 cyclin-dependent kinase: chemical library screen and antileishmanial activity

The CRK3 cyclin-dependent kinase of Leishmania has been shown by genetic manipulation of the parasite to be essential for proliferation. We present data which demonstrate that chemical inhibition of CRK3 impairs the parasite's viability within macrophages, thus further validating CRK3 as a potential drug target. A microtiter plate-based histone H1 kinase assay was developed to screen CRK3 against a chemical library enriched for protein kinase inhibitors. Twenty-seven potent CRK3 inhibitors were discovered and screened against Leishmania donovani amastigotes in vitro. Sixteen of the CRK3 inhibitors displayed antileishmanial activity, with a 50% effective dose (ED50) of less than 10 microM. These compounds fell into four chemical classes: the 2,6,9-trisubstituted purines, including the C-2-alkynylated purines; the indirubins; the paullones; and derivatives of the nonspecific kinase inhibitor staurosporine. The paullones and staurosporine derivatives were toxic to macrophages. The 2,6,9-trisubstituted purines inhibited CRK3 in vitro, with 50% inhibitory concentrations ranging from high nanomolar to low micromolar concentrations. The most potent inhibitors of CRK3 (compounds 98/516 and 97/344) belonged to the indirubin class; the 50% inhibitory concentrations for these inhibitors were 16 and 47 nM, respectively, and the ED50s for these inhibitors were 5.8 and 7.6 microM, respectively. In culture, the indirubins caused growth arrest, a change in DNA content, and aberrant cell types, all consistent with the intracellular inhibition of a cyclin-dependent kinase and disruption of cell cycle control. Thus, use of chemical inhibitors supports genetic studies to confirm CRK3 as a validated drug target in Leishmania and provides pharmacophores for further drug development.

Production of a soluble cyclin B/cdc2 substrate for cdc25 phosphatase

Study of the function and regulation of the important cell cycle regulator cdc25 phosphatase has been hampered by the lack of a sensitive and specific substrate and assay. Here we report the production of a specific and sensitive substrate for the cdc25 phosphatase. The substrate is human cyclin B1/cdc2 phosphorylated on the inhibitory Thr14 and Tyr15 residues and activating Thr161 on cdc2, and is relatively simple to produce from readily available materials. The assay is based on the cdc25-specific dephosphorylation and activation of the phosphorylated cyclin B1/cdc2 substrate (PY15), using the increased histone H1 kinase activity of the activated PY15 as a read-out of cdc25 activity.

Studies on the in vitro phosphorylation of HSSB-p34 and -p107 by cyclin-dependent kinases. Cyclin-substrate interactions dictate the efficiency of phosphorylation

Cyclin-dependent kinases (Cdks) are required for cell cycle progression. Two potentially significant Cdk substrates in human cells are the human single-stranded binding protein (HSSB or RPA), which plays an essential role in DNA replication, repair, and recombination, and the tumor suppressor p107 which acts to negatively regulate cell growth. In this report we describe the in vitro phosphorylation of these two proteins by Cdks in an attempt to understand how cyclin-substrate interactions direct phosphorylation efficiencies. We show that cyclin A-Cdk2 efficiently phosphorylates the p34 subunit of HSSB (HSSB-p34) alone or as a part of the heterotrimeric complex. In contrast, cyclin E-Cdk2 that is active in phosphorylating histone H1, does not support the phosphorylation of the p34 subunit of HSSB. We provide evidence that this differential phosphorylation results from a specific interaction between HSSB-p34 and cyclin A, but not cyclin E. Thus the observed cell cycle-dependent phosphorylation of HSSB-p34 at the G1 to S transition is most likely catalyzed by cyclin A-Cdk2 initiated by the direct interaction between cyclin A and the HSSB-p34 subunit. These studies are consistent with our previous observation that p107, which directly binds cyclin A, is efficiently phosphorylated by cyclin A-Cdk2 but not cyclin B-associated kinases. Here we further demonstrate that cyclin A only complexes with p107 in its unphosphorylated form. These data suggest a catalytic mechanism by which Cdk acts: substrate targeting by a cyclin-substrate interaction followed by dissociation of the Cdk upon phosphate incorporation allowing the Cdk to become available for the next cycle of phosphorylation.

Comparison of acute hepatocellular proliferating cell nuclear antigen labeling indices and growth fractions, p34cdc2 kinases, and serum enzymes in carbon tetrachloride-treated rats

We evaluated various biomarkers associated with cell proliferation immediately following insult with the classic hepatotoxicant carbon tetrachloride (CCl4). Rats were administered a single necrogenic dose of CCl4 and euthanized at either t = 4, 8, 12, 16, or 24 hr postdose. Parameters evaluated included the following: immunohistochemical detection of hepatocellular proliferating cell nuclear antigen labeling indices (PCNA-LIs; percentage of cells in S phase) and growth fractions (PCNA-GFs; percentage of cells in the cell cycle); PCNA and the cyclin-dependent kinase p34cdc2 (CDK) protein in S-9 fractions by Western blot and enzyme-linked immunosorbent assay (ELISA); and liver-related serum enzymes. An increase in PCNA-GF was observed at t = 4 hr, concomitant with elevations in CDK and PCNA protein (Western blot). PCNA-LIs were increased by t = 24 hr, as were CDK and PCNA by ELISA. Sorbitol dehydrogenase was the most sensitive enzyme, with increases observed at t = 4 hr. Our results indicate that PCNA-GF, CDK, and PCNA levels reflect hepatocellular regeneration as early as 4 hr following CCl4 insult. We conclude that these assays are early and sensitive indicators of acute hepatotoxicity that may be advantageous to evaluate in the early stages of exploratory studies.

The role of the 34-kDa subunit of human replication protein A in simian virus 40 DNA replication in vitro

Human replication protein A (RPA) is a three subunit protein complex involved in DNA replication, repair, and recombination. We investigated the role of the 34-kDa subunit (p34) of RPA in DNA replication by generating a series of p34 mutants. While deletion of the N-terminal domain of p34 prevented its phosphorylation by both cyclin-dependent kinase (Cdk) and DNA-dependent kinase, a double point mutant that lacks the major phosphorylation sites for Cdk could be phosphorylated by DNA-dependent kinase. In simian virus 40 (SV40) DNA replication, RPA containing either of these mutants functioned as efficiently as wild-type RPA. However, mutant RPA containing C-terminally deleted p34 was only marginally active. This indicates that the C-terminal region, but not the phosphorylation domain of p34, is necessary for RPA function in DNA replication. Furthermore, RPA containing the C-terminally deleted p34 mutant could stimulate DNA polymerase alpha, and bind to single-stranded DNAs but was limited in its ability to unwind DNA or interact with SV40 large T antigen (T Ag). These results suggest that RPA p34 interacts with SV40 T Ag during the initiation of SV40 DNA replication and may be necessary for DNA unwinding.