p34 Kinase-mediated release of lamins from nuclear ghosts is inhibited by cAMP-dependent protein kinase

During mitosis the lamins are found in a hyperphosphorylated and soluble state. p34^cdc2 kinase (MPF), a protein kinase complex with a pivotal role during mitosis, has been found to phosphorylate the lamins and, in some cases, though not all, to cause depolymerization of the lamina in vitro. Due to the variety of protein interactions in the lamina, there is a probable requirement for multiple enzyme activities to effect its breakdown in mitosis. Using nuclear ghosts as substrate, we have fractionated a Xenopus mitotic extract into a lamin-releasing fraction (p34^cdc2 kinase) and a fraction that inhibits p34^cdc2 kinase-mediated lamin release if the nuclear ghosts are first preincubated in it. The lamin-release-inhibiting activity in the p34^cdc2 kinase-depleted mitotic extract is, in turn, inhibited if PKI, a protein kinase inhibitor specific for PKA, is included in the preincubation reaction mixture. Furthermore, a similar degree of inhibition can be achieved by using purified PKA to preincubate the nuclear ghosts. This suggests that dephosphorylation of PKA substrate sites is necessary for lamin depolymerization.