THE NUCLEAR PHOSPHOINOSITIDE CYCLE - DOES IT PLAY A ROLE IN NUCLEAR CA2+ HOMEOSTASIS

The probable answer to this question is no. Much of the current eviden ce summarised elsewhere in this issue points to nuclear Ca2+ changes c hanging in response to cytosolic Ca2+ with little evidence for an inde pendently controlled nuclear Ca2+ homeostasis. There are InsP(3) recep tors in the nuclear membrane, and it is possible that during nuclear m embrane assembly the InsP(3) acting on these (Sullivan and Wilson, thi s issue) is formed by an inositide cycle located on the assembling nuc lear skeleton. But our current experimental data suggest that when the nucleus is intact, InsP(3) generated by this cycle would have to exit through the nuclear pores to act on any known InsP(3) receptors. Thus the nuclear inositide cycle appears more likely to serve to generate diacylglycerol to activate protein kinase C, and/or to generate inosit ol phosphates such as InsP(2), which may have distinct intranuclear fu nctions.