Calyculin Research Articles

Okadaic Acid as a Probe to Analyse the Cell Cycle Progression in Plant Cells

AbstractPreviously we have demonstrated the dynamic change of microtubules (MTs) during cell cycle progression using highly synchronized tobacco BY‐2 cells and characterized the specific transition points of MT organization (Hasezawa and Nagata, 1991). In this study the effect of okadaic acid (OA), a specific inhibitor of protein phosphatase 1 and 2A, on such changes of MTs during cell cycle was examined. These experiments revealed that cell cycle was arrested before the formation of the preprophase band (PPB), at anaphase and at the border of M/G1. Although the block at the anaphase seemed to be analogous to that observed in animal cells (Yamashita et al., 1990), the other two blocks were specific to plant cells. It is interesting that these two blocks coincided with the transition points of MT organization, as revealed in the previous study (Hasezawa and Nagata, 1991). Thus it is proposed that phosphorylation is involved in MT organization, although the effect of OA has been shown mainly to be the activation of cdc‐2/histone H1 kinase in animal cells. Another inhibitor of protein phosphatase 1 and 2A, calyculin A (CLA), showed very similar effects on the cell cycle progression. The use of such inhibitors to dissect the cell cycle progression of plant cells is discussed.

Calyculin A modulates voltage-dependent outward current in guinea-pig taenia coli smooth muscle cells.

Protein kinases modulate the electrical activity of various cells through their action on membrane ion channels. In cardiac and smooth muscle cells, enhancement of a number of K+ currents by β-adrenergic agonists, such as isoproterenol (ISP), may result from a direct phosphorylation of K+ channel protein by cAMP-dependent protein kinase (A kinase) (Kolb, 1990). On the other hand, calyculin A (CL-A) which is a toxin isolated from marine sponge (Discodermia calyx) has a potent inhibitory action on protein phosphatase activities (Ishihara et al., 1989).

Calyculin A increases voltage-dependent inward current in smooth muscle cells isolated from guinea pig taenia coli.

The effects of a potent phosphatase inhibitor, calyculin A (CL-A), on inward currents in guinea pig taenia coli smooth muscle cells were examined. CL-A increased the inward current, and this effect of CL-A was inhibited by a protein kinase C inhibitor, H-7, and by nifedipine. Phorbol 12,13-dibutyrate, an activator of protein kinase C, also increased the inward current and this effect was antagonized by H-7. These results suggest that in guinea pig taenia coli smooth muscle cells CL-A may facilitate the opening of the L-type Ca2+ channels through the protein kinase C-dependent phosphorylation system.