Association of DNA with the nuclear lamina in Ehrlich ascites tumor cells

We have studied in vitro binding of DNA to nuclear lamina structures isolated from Ehrlich ascites tumor cells. At low ionic strength in the presence of Mg ++ , they bind considerable amounts of mouse and bacterial DNA, forming complexes stable in 2 M NaCl. Single‐stranded DNA and pulse‐labeled DNA show higher binding efficiencies than native uniformly labeled DNA. When mixing occurs in 2 M NaCl, complex formation is inhibited. When nuclei are digested with DNAse I under conditions that favor chromatin condensation, DNA associated with matrices subsequently prepared from such nuclei is markedly enriched in satellite DNA. If digestion is carried out with DNAse II while nuclei are decondensed in EDTA, no enrichment in satellite DNA is observed. Preparations of purified, high‐molecular weight, double‐stranded DNA contain variable amounts of fast‐sedimenting aggregates, which are insoluble in 2 M NaCl but are dispersed by DNA fragmentation or denaturation. These results point at some artifacts inherent in studies of DNA bound to residual nuclear structures in vivo and suggest conditions expected to avoid these artifacts. Further, using controlled digestion with DNAse II, we have studied the in vivo association of DNA with nuclear lamina isolated from Ehrlich ascites tumor cells. In the course of DNA fragmentation from above 50 kbp to about 20 kbp average size, the following events were observed. The DNA of high molecular weight (much longer than 50 kbp) behaved as if tightly bound to the nuclear lamina, as judged by sedimentation in sucrose and metrizamide density gradients, electron microscopy, and retention on glass fiber filters. As the size of DNA decreased, it was progressively detached from the nuclear lamina, and at about 20 kbp average, length practically all DNA was released. The last 1–4% of DNA, although cosedimenting with the nuclear lamina in sucrose gradients, behaved as free DNA, banding at 1.14 g/cm 3 in metrizamide density gradients and showing less than 4% retention on filters. At no stage of digestion did the DNA cosedimenting with nuclear lamina show changes in satellite DNA content relative to that of total DNA or enrichment in newly replicated DNA. It was shown, however, that digestion of nuclear lamina‐DNA complex with EcoRI or Hae III led to the formation of DNA‐protein aggregates, which banded at 1.35 g/cm 3 in high salt containing metrizamide density gradients and which were strongly enriched in satellite DNA. These results argue against the existence of direct tight bonds between DNA and nuclear lamina in vivo but demonstrate that such bonds can be generated under certain conditions in vitro.