Abstract. Cell-free extracts prepared from activated clam oocytes contain factors which induce phosphorylation of the single 67-kD lamin (L67), disassemble clam oocyte nuclei, and cause chromosome condensation in vitro (Dessev, G., R. Palazzo, L. Rebhun, and R. Goldman. 1989. Dev. Biol. 131:469-504). To identify these factors, we have fractionated the oocyte extracts. The nuclear lamina disassembly (NLD) activity, together with a protein kinase activity specific for L67, appear as a single peak throughout a number of purification steps. This peak also contains p34 ¢dc2, cyclin B, and histone Hl-kinase activity, which are components of the M-phase promoting factor (MPF). The NLD/L67-kinase activity is depleted by exposure of this purified material to Sepharose conjugated to p13 =n¢~, and is restored upon addition of a p34~d°2/p62 complex from HeLa cells. The latter complex phosphorylates L67 and induces NLD in the absence of other clam oocyte proteins. Our results suggest that a single protein kinase activity (p34~c2-H1 kinase, identical with MPF) phosphorylates the lamin and is involved in the meiotic breakdown of the nuclear envelope in clam oocytes.

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A nuclear lamina-enriched fraction from Ehrlich ascites tumor cells contains a tightly bound protein kinase activity, which phosphorylates in vitro the nuclear lamins, a 52-kilodalton protein, and several unknown minor components. The enzyme(s) is thermolabile, independent of Ca2+ and cAMP, and inhibited by quercetin. After treatment with 4 M urea it remains bound to the nuclear lamina in an active state, but it is irreversibly inactivated in 6 M urea. The lamin proteins are phosphorylated on serine residues. Their two-dimensional phosphopeptide maps show multiple phosphorlation sites and a considerable similarity to the phosphopeptide maps of lamins labeled in vivo. Photoaffinity labeling experiments revealed several polypeptide fractions in the nuclear lamina fraction that are candidates for the protein kinase(s).The nuclear lamina (NL) is a proteinaceous meshwork located at the inner surface of the nuclear membrane (1, 2). The major constituents of the NL are three polypeptides with molecular masses between 60 and 75 kDa, termed lamins A, B, and C (2). It is thought that the NL plays a role in maintaining the structural organization of both the nuclear envelope (3, 4) and chromatin (5, 6) in the interphase nucleus. It has also been shown that the lamins are members of the intermediate filament protein family (7-11).The nuclear lamins are phosphoproteins. During interphase they exhibit a relatively low level of phosphorylation of about 0.2 mol of phosphate per mol of protein (12). However, when the NL is disassembled in mitosis (3, 4) there is an increased phosphorylation of the three lamins (4, 12). Little is known about the nature, location, and regulation of the protein kinase(s) involved in both interphase and mitotic phosphorylation of the lamins.We have recently developed a method for the isolation of the NL from Ehrlich ascites tumor (EAT) cells (13,14 (Amersham) at 25 uCi/ml in DME otherwise free of phosphate or methionine, respectively, supplemented with 5% dialyzed fetal calf serum.The original method developed for the isolation of NL from EAT cells (13, 14) was modified as follows. One milliliter of packed cells was resuspended in 30 tnl of solution 1 (0.25 M sucrose/5 mM EDTA/5 mM Tris HCI, pH 7.5/0.5 mM phenylmethylsulfonyl fluoride), incubated at 0C for 5 min, and centrifuged at 600 x g for 5 min. The cells were treated once more with 30 ml of solution I and twice with 30 ml of solution 11 (0.25 M sucrose/0.1 mM EDTA/5 mM Tris-HCI, pH 7.5/0.5 mM phenylmethylsulfonyl fluoride). Each time the cells were resuspended (Vortex mixer), incubated for 5 min at 0C, and collected by centrifugation at 600 x g for 5 min. The pellet was then resuspended in 10 ml of solution II, mixed with 10 ml of aqueous 1% Nonidet P-40 (Sigma), layered over a 60-ml cushion of solution III (0.25 M sucrose/0.1 mM EDTA/5 mM Tris HCl, pH 7.0), and spun at 3500 x g for 15 min. The resulting semitransparent pellet was resuspended in 10 ml of solution III and incubated for 30 min at room temperature in the presence of DNa...