Specific phosphorylation of proteins in pore complex‐laminae from the sponge Geodia cydonium by the homologous aggregation factor and phorbol ester. Role of protein kinase C in the phosphorylation of DNA topoisomerase II.

We have recently shown that the aggregation factor (AF) from the sponge Geodia cydonium stimulates DNA synthesis in quiescent, dissociated cells from the same organism; this event was correlated with the release of the two second messengers: inositol trisphosphate and diacylglycerol. Here we describe that after binding of the AF to the plasma membrane‐bound aggregation receptor, a rapid and drastic increase in the incorporation of 32Pi into a series of proteins in the pore complex‐lamina fraction occurs. Addition of the tumor promoter, 12‐O‐tetradecanoylphorbol‐13‐acetate, to quiescent cells resulted in a similar stimulation of phosphorylation of nuclear proteins. Among them we have selected one protein with a polypeptide Mr of 170,000 (pp170) for detailed studies. By immunoblotting pp170 was identified as DNA topoisomerase II. In vitro studies with nuclei and purified, homogeneous protein kinase C together with the required activators of this enzyme also showed a phosphorylation of pp170. After phosphorylation, DNA topoisomerase II activity was found to be 2.5‐fold that of the non‐phosphorylated enzyme. From these data we conclude that protein kinase C is involved in AF induced transmembrane signalling, ultimately leading to an initiation of DNA synthesis.