Phosphoinositidase C isoforms are specifically localized in the nuclear matrix and cytoskeleton of Swiss 3T3 cells.

The fine subcellular localization of different phosphoinositidase C (PIC) isoforms has been determined by both electron microscope immunocytochemistry and immunoblotting in whole Swiss 3T3 cells as well as in subcellular fractions. PIC-beta, whose signaling activity has been recently demonstrated at the nuclear level (M. A. Martelli, R. S. Gilmour, V. Bertagnolo, L. M. Neri, L. Manzoli, and L. Cocco, Nature, 358, 242-244, 1992), is mainly localized in the interchromatin domains, while it is almost absent from the cytoplasm. PIC-gamma is almost absent from the nucleus and resides in the cytosol, while PIC-delta is undetectable in these cells. PIC-beta is retained in the inner nuclear matrix and lacks the nuclear pore-lamina complex, whereas PIC-gamma is preferentially associated with cytoskeletal filaments. Moreover, PIC-beta is present at the same sites of the nuclear matrix where phospholipids and protein kinase C can be identified. This indicates that some elements of the phosphoinositide signal transduction system are located inside the nucleus. Moreover, PIC-gamma association with the cytoskeleton filaments suggests a possible involvement of this enzyme in cytoskeleton-mediated changes of cell shape.