NUCLEAR-ENVELOPE BREAKDOWN IN MAMMALIAN-CELLS INVOLVES STEPWISE LAMINA DISASSEMBLY AND MICROTUBULE-DRIVEN DEFORMATION OF THE NUCLEAR-MEMBRANE

We have studied nuclear envelope disassembly in mammalian cells by mor phological methods. The first signs of nuclear lamina depolymerization become evident in early prophase as A-type lamins start dissociating from the nuclear lamina and diffuse into the nucleoplasm. While B-type lamins are still associated with the inner nuclear membrane, two symm etrical indentations develop on antidiametric sites of the nuclear env elope, These indentations accommodate the sister centrosomes and assoc iated astral microtubules, At mid-to late prophase, elongating microtu bules apparently push on the nuclear surface and eventually penetrate the nucleus, At this point the nuclear envelope becomes freely permeab le to large ligands, as indicated by experiments with digitonin-treate d cells and by the massive release of solubilized A-type lamins into t he cytoplasm, At the prophase/prometaphase transition, the B-type lami na is fragmented, but 'islands' of lamin B polymer can still be discer ned on the tips of congressing chromosomes. Finally, at metaphase, the lamin B polymer breaks down into small pieces, which tend to concentr ate in the area of the mitotic spindle, Nuclear envelope breakdown is not prevented when the microtubules are depolymerized by nocodazole; h owever, the mode of nuclear lamina fragmentation in the absence of mic rotubules is markedly different from the normal one and involves multi ple raffles and gaps, which develop rapidly along the entire surface o f the nuclear envelope. These data suggest that nuclear envelope disas sembly is a stepwise process in which the microtubules play an importa nt part.