Identification of p34cdc2 kinase from sea urchin Hemicentrotus pulcherrimus and its involvement in the phosphorylation of myosin II regulatory light chain in the metaphase extract

The phosphorylation of myosin II at the Ser1 and Ser2 is critical for normal platelet-derived growth factor induced reorganization of myosin filaments

Phosphorylation of the regulatory light chain of myosin II (MLC(20)) at the activation sites promotes both the motor activity and the filament formation of myosin II, thus playing an important role in various cell motile processes. In contrast, the physiological function of phosphorylation of MLC(20) at the inhibitory sites is unknown. Here we report for the first time the function of the inhibitory site phosphorylation in the cells. We successfully produced the antibodies specifically recognizing the phosphorylation sites of MLC(20) at Ser1, and the platelet-derived growth factor (PDGF)-induced change in the phosphorylation at the Ser1 was monitored. The phosphorylation of MLC(20) at the Ser1 significantly increased during the PDGF-induced actin cytoskeletal reorganization. PDGF disassembled the stress fibers, and this was attenuated with the expression of unphosphorylatable MLC(20) at the Ser1/Ser2 phosphorylation sites. The present results suggest that the down-regulation of myosin II activity achieved by the phosphorylation at the Ser1/Ser2 sites plays an important role in the normal reorganization of actomyosin filaments triggered by PDGF receptor stimulation.

Calcium-responsive contractility during fertilization in sea urchin eggs

Fertilization triggers a reorganization of oocyte cytoskeleton, and in sea urchins, there is a dramatic increase in cortical F-actin. However, the role that myosin II plays during fertilization remains largely unexplored. Myosin II is localized to the cortical cytoskeleton both before and after fertilization and to examine myosin II contractility in living cells, Lytechinus pictus eggs were observed by time-lapse microscopy. Upon sperm binding, a cell surface deflection traversed the egg that was followed by and dependent on the calcium wave. The calcium-dependence of surface contractility could be reproduced in unfertilized eggs, where mobilization of intracellular calcium in unfertilized eggs under compression resulted in a marked contractile response. Lastly, inhibition of myosin II delayed absorption of the fertilization cone, suggesting that myosin II not only responds to the same signals that activate eggs but also participates in the remodeling of the cortical actomyosin cytoskeleton during the first zygotic cell cycle.

Role of myosin light chain phosphorylation in the regulation of cytokinesis

Phosphorylation of regulatory light chain (RMLC) of myosin II at Ser19/Thr18 is likely to play important roles in controlling the morphological changes seen during cell division of cultured mammalian cells. Phosphorylation of RMLC regulates the activity of myosin II, an essntial motor for cytokinesis, and phosphorylation of RMLC shows dramatic changes during mitosis. Two exzymes, myosin phosphatase and kinase, control phosphorvlation of RMLC. Myosin phosphatase is activated during mitosis, apparently as a result of mitosis-specific phosphorylation of the myosin phosphatase targeting subunit (MYPT). This activation of myosin phosphatase is likely to result in RMLC dephosphorylation, causing the disassemly of stress fibers and focal adhesions during prophase. The phosphorylation of MYPT is lost in cyotokinesis, which would decrease myosin phosphatase activity. At the same time, ROCK (Rho-kinase) probably phosphorylates MYPT at its inhibitory sites, further decreasing the activity of myosin phosphatase. These changes in MYPT phosphorylation would raise RMLC phosphorylation, leading to the activation of myosin II for cyotokinesis. RMLC phosphorylation is also regulated by several RMLC kinases including ROCK (Rho-kinase), MLCK and citron kinase, all of which are localized at cleavage furrows. Future studies should examine whether these multiple kinases are redundant or whether they control distinct aspects of cell division.

A PSTTLRE-form of cdc2-like gene in the marine microalga Dunaliella tertiolecta

To understand the genetic control of algal cell division cycle that pertains to phytoplankton bloom dynamics in the sea, we cloned and analyzed a gene coding for a cyclin-dependent kinase (CDK) for the chlorophyte Dunaliella tertiolecta. The cDNA cloned, 1061 bp long, contained an open reading frame of 314 amino acids. FASTA and GAP analyses showed that this sequence was most homologous to cdc2 out of all known cdks, with an identity of 54-68% and a similarity of 65-76% to cdc2 in higher plants, animals, and yeast. Several signature domains of cdc2 were identified from this sequence, although the PSTAIRE and GDSEID motifs were replaced with PSTTLRE and GDCELQ, respectively. Southern blot hybridization demonstrated that this gene occurred as a single copy in this species, and quantitative RT-PCR showed that the transcription of this gene was constitutive. The present results suggest that the universal cdc2 is conserved in the lower eukaryote with unique structural characteristics.

Butyrolactone I induces cyclin B1 and causes G2/M arrest and skipping of mitosis in human prostate cell lines

Several naturally occurring cyclin-dependent kinase (CDK) inhibitors have been isolated from different lower organisms. In this report, we examined the effect of one of the CDK inhibitors, butyrolactone I (BL), on the expression of cyclins D2, A and B1 in three human prostatic cancer cell lines (DU145, PC-3, LNCaP) using two colored flow cytometric analysis. The percentage of DU145 cells in the 4C phase of the cell cycle were increased significantly at both 70 microM and 100 microM BL. Furthermore, an additional 8C peak was observed which had double the DNA content of the 4C phase at these concentrations of BL. The appearance of the 8C peak increased gradually and was more evident in DU145 and PC-3 than LNCaP. Cells in the 8C peak had either two nuclei or abnormal nuclei as observed by Papanicolaou stain. BL also increased the amount of cyclin B1 positive cells in the 4C phase. This increase was apparent on day 1 and returned to normal by day 3. Since BL selectively inhibits cyclin-dependent kinase, cyclin B1 might accumulate without being degraded. Other cyclins were not significantly changed by BL. The data demonstrate that BL inhibited Cdc2 of unsynchronized cultured prostate cancer cells, and interrupted the cell cycle progression toward cell division. The BL inhibition of Cdc2 led to the accumulation of cells in the 4C phase without mitosis resulting in an accumulation of cyclin B1. The appearance of cells in the 8C phase may be due to the progression of cells in the 4C phase through the cell cycle skipping mitosis. Cyclin B1 decreased in correlation with the progression through a new cell cycle. These results suggest that BL does not cause a complete arrest of the cell cycle in G2/M but that BL occasionally allows for the skipping of mitosis and subsequent progression through the cell cycle to occur.