Neuronal nuclear acetyltransferases involved in the synthesis of platelet-activating factor are located in the nuclear envelope and show differential losses in activity

Neuronal nuclear fraction N-1 was isolated from cerebral cortices of 15-day-old rabbits, and nuclear subfractions prepared, in order to study the location of nuclear lyse platelet-activating factor (lyso-PAF) acetyltransferase and alkylglycerophosphate (AGP) acetyltransferase, and factors that affect the loss of these two nuclear activities. Subfractionation of prelabelled N-1 indicated that the nuclear envelope had the highest percentage of the radioactive acetylated products alkylacetylglycerophosphate (AAGP) and PAF, and the distribution of these phospholipids reflected phospholipid distributions in the nuclear subfractions. The majority (95%) of radioactive AAGP and PAF was also recovered in Triton X-100 extracts of prelabelled nuclei, suggesting that these acetylated lipids are located in nuclear membranes rather than in the nuclear matrix/chromatin. Of the nuclear subfractions, the envelope had the highest AGP and lyso-PAF acetyltransferase specific activities which were close to corresponding values seen in the parent N-1 fraction. Thus the nuclear AGP and lyso-PAF acetyltransferases were principally localized to the nuclear membranes. Differentials in activity loss were seen for the two acetyltransferase activities. In the nuclear envelope fractions, the lyso-PAF acetyltransferase was the more susceptible to oxidation reactions which could be reversed or blocked by the use of reducing agents. In preincubations, N-1 showed greater losses in lyso-PAF acetyltransferase activity than in AGP acetyltransferase activity, losses which were not attributable to oxidation. Addition of cytosolic fraction S-3 to preincubations promoted losses for each acetyltransferase in N-1, and gave evidence for cytosolic and endogenous nuclear contributions to the activity loss. Addition of okadaic acid to the preincubations did not prevent the decline of either acetyltransferase in intact nuclei, but did diminish the loss of nuclear lyso-PAF acetyltransferase activity promoted by S-3 addition, and also blocked the loss of this acetyltransferase seen in preincubations of isolated nuclear envelopes. This suggests that nuclear lyso-PAF acetyltransferase is susceptible to okadaic acid-sensitive nuclear and cytosolic protein phosphatase activities, while AGP acetyltransferase may lose activity by the action of other phosphatases or by other mechanisms within the nucleus.