DIFFERENTIAL MITOTIC PHOSPHORYLATION OF PROTEINS OF THE NUCLEAR-PORE COMPLEX

During each cell cycle, the nucleus of higher eukaryotes undergoes a d ramatic assembly and disassembly, These events can be faithfully repro duced in vitro using cell-free extracts derived from Xenopus eggs, Suc h extracts contain three major N-acetylglucosaminylated proteins, p200 , p97, and p60, Ah three become assembled into reconstituted nuclear p ores, Here we show that p200, p97, and p60 exist in eggs in soluble hi gh molecular mass complexes of 1000, 450, and 600 kDA, respectively, T he bulk of p60 is stably associated with proteins of 58 and 54 kDa, wh ile p200 is associated with a fraction of p60 in a separate complex la cking p58 and p54. Upon examining the behavior of these proteins in th e cell cycle, we find that p200 and p97 are highly phosphorylated at m itosis, both in vivo and in vitro, Moreover, in extracts that cycle be tween interphase and mitosis, p200 and p97 are specifically phosphoryl ated at mitosis. Corresponding with their mitotic phosphorylation, bot h p200 and p97 are specific substrates for purified mitotic Cdc2 kinas e, whereas nucleoporin p60 is not, Analysis indicates that the size of the complexes containing the pore N-acetylglucosamine glycoproteins d oes not change during mitosis, suggesting that such complexes represen t stable multicomponent modules into which the nucleus disassembles at mitosis.