CAMP-DEPENDENT PROTEIN-KINASE PHOSPHORYLATES AND ACTIVATES NUCLEAR CA2-ATPASE()

A Ca2+-pump ATPase, similar to that in the endoplasmic reticulum, has been located on the outer membrane of rat liver nuclei, The effect of cAMP-dependent protein kinase (PKA) on nuclear Ca2+-ATPase (NCA) was s tudied by using purified rat liver nuclei, Treatment of isolated nucle i with the catalytic unit of PKA resulted in the phosphorylation of a 105-kDa band that was recognized by antibodies specific for sarcoplasm ic reticulum Ca2+-ATPase type 2b. Partial purification and immunoblott ing confirmed that the 105-kDa protein band phosphorylated by PKA is N CA. The stoichiometry of phosphorylation was 0.76 mol of phosphate inc orporated/moI of partially purified enzyme. Measurement of ATP-depende nt Ca-45(2+) uptake into purified nuclei showed that PKA phosphorylati on enhanced the Ca2+-pumping activity of NCA. We show that PKA phospho rylation of Ca2+-ATPase enhances the transport of 10-kDa fluorescent-l abeled dextrans across the nuclear envelope. The findings reported in this paper are consistent with the notion that the crosstalk between t he cAMP/PKA- and Ca2+-dependent signaling pathways identified at the c ytoplasmic level extends to the nucleus. Furthermore, these data suppo rt a function for crosstalk in the regulation of calcium-dependent tra nsport across the nuclear envelope.