Cell division and motility are generated by the cytoskeletal structures contained in networks of actin filaments and microtubules. Among the Ca2+-activated protein kinases, protein kinase C (PKC) can be activated with tumor promoter, 12-O-tetradecanoyl phorbol-13-acetate (TPA) (Kikkawa et al., 1983), which induces alterations in the cytoskeleton of cultured cells. To study the regulation of the cytoskeleton by PKC, we investigated the formation of actin filaments, microtubules, changes in cell shape and Ca2+ signalling after treatment with TPA in unfertilised eggs of the sea urchin Hemicentrotus pulcherrimus.

Formation of actin filaments and microtubles was induced in the eggs by the TPA treatment. When the eggs were treated with an inactive TPA, 4α-PMA, these cytoskeletal changes did not occur. Moreover, elongation of actin filaments was inhibited by the Ca2+ chelating agent BAPTA, but microtubule organisation was unaffected by intracellular Ca2+ chelation in the TPA-treated egg. TPA is known to induce alkalisation in sea urchin eggs (Swann & Whitaker, 1985) and when the eggs are treated with TPA in Na+-free seawater, the rise in intracellular pH (pHi) is depressed. However, elongation of actin filaments and microtubules occurred under these conditions. An increase in pHi is considered to be essential for triggering the bundle of actin filaments (Begg et al., 1982) and microtubule assembly (Schatten et al., 1992). These results show that actin and microtubule assemblies require activation of PKC, and that the formation of actin and microtubules may be related to the type of PKC isoform: Ca2+-dependent or -independent. Moreover, this mechanism is essentially independent of cytoplasmic alkalisation.