Grue P, Grässer A, Sehested M, Jensen P B, Uhse A, Straub T, Ness W, Boege F
Medizinische Poliklinik, University of Würzburg, Klinikstrasse 6-8, D-97070 Würzburg, Germany.
J Biol Chem. 1998 Dec 11;273(50):33660-6. doi: 10.1074/jbc.273.50.33660.
Unique functions of mammalian DNA-topoisomerases IIalpha and -beta are suggested by their distinct cellular distribution and chromatin binding at mitosis. Here, we studied H69-VP cells that, due to a homozygous mutation, express topoisomerase IIalpha mostly outside the nucleus. In these cells topoisomerase IIbeta showed a normal nuclear localization. However, at mitosis it diffused away from the chromatin despite the nuclear lack of the alpha-isoform. 80% of these cells performed chromosome condensation and disjunction with the aid of cytosolic topoisomerase IIalpha, which bound to the mitotic chromatin with low affinity. However, the genotype of these cells was highly polyploid indicating an increased rate of non-disjunction. In 20% of the mutant cells neither topoisomerase II isoform was bound to the mitotic chromatin, which appeared as an unstructured DNA spheroid unable to undergo disjunction and cytokinesis. Parental H69 cells expressing topoisomerase IIalpha inside the nucleus exhibited high affinity binding of the enzyme to the mitotic chromatin. Their genotype was mostly diploid and stable. We conclude (i) that high affinity chromatin binding of topoisomerase IIalpha is essential for chromosome condensation/disjunction and (ii) that topoisomerase IIbeta does not adopt these functions.
哺乳动物DNA拓扑异构酶IIα和IIβ的独特功能是由它们在有丝分裂时不同的细胞分布和染色质结合所暗示的。在这里，我们研究了H69-VP细胞，由于纯合突变，这些细胞的拓扑异构酶IIα大多在细胞核外表达。在这些细胞中，拓扑异构酶IIβ显示出正常的核定位。然而，在有丝分裂时，尽管细胞核中缺乏α异构体，它仍从染色质上扩散开来。80%的这些细胞借助于与有丝分裂染色质低亲和力结合的胞质拓扑异构酶IIα进行染色体浓缩和解离。然而，这些细胞的基因型高度多倍体，表明不分离率增加。在20%的突变细胞中，两种拓扑异构酶II异构体都未与有丝分裂染色质结合，有丝分裂染色质表现为无法进行解离和胞质分裂的无结构DNA球体。在细胞核内表达拓扑异构酶IIα的亲代H69细胞中，该酶与有丝分裂染色质表现出高亲和力结合。它们的基因型大多是二倍体且稳定。我们得出结论：（i）拓扑异构酶IIα与染色质的高亲和力结合对于染色体浓缩/解离至关重要；（ii）拓扑异构酶IIβ不具备这些功能。
