The localization of LAP2 beta during pronuclear formation in bovine oocytes after fertilization or activation.

We have shown that the assembly of lamin-associated polypeptide (LAP) 2beta was detected surrounding the chromatin mass around the time of extrusion of the second polar body (PB) in some fertilized oocytes, but not in most activated oocytes, by using A23187 and cycloheximide (CaA + CH). Here, we immunohistologically analysed the correlation between LAP2beta assembly and chromatin condensation in fertilized and activated oocytes during the second meiosis. In bovine cumulus cells, the onset of LAP2beta assembly was observed around anaphase chromosomes with strongly phosphorylated histone H3. No LAP2beta assembled around the chromosomes in the first and second polar bodies and the alternative oocyte chromatin (oCh) if histone H3 was phosphorylated. Only histone H3 of oCh was completely dephosphorylated during the telophase II/G1 transition (Tel II/G1), and then LAP2beta assembled around only the oCh without phosphorylated histone H3. In the oocytes activated by CaA + CH, LAP2beta did not assemble around the condensed oCh during the Tel II/G1 transition, although their histone H3 dephosphorylation occurred rather rapidly compared with that of the fertilized oocytes. The patterns of histone H3 dephosphorylation and LAP2beta assembly in oocytes activated by CaA alone showed greater similarity to those in fertilized oocytes than to those in oocytes activated by CaA + CH. These results show that LAP2beta assembles around only oCh after complete dephosphorylation of histone H3 after fertilization and activation using CaA alone, and that the timing of histone H3 dephosphorylation and LAP2beta assembly in these oocytes is different from that of somatic cells. The results also indicate that CH treatment inhibits LAP2beta assembly around oCh but not histone H3 dephosphorylation.