What drives nuclear growth? Studying nuclei assembled in Xenopus egg extract and focusing on
importin α/β-mediated nuclear import, we show that, while import is required for nuclear growth, nuclear growth and import can be uncoupled when
chromatin structure is manipulated. Nuclei treated with
micrococcal nuclease to fragment
DNA grew slowly despite exhibiting little to no change in import rates. Nuclei assembled around axolotl
chromatin with 20-fold more
DNA than Xenopus grew larger but imported more slowly. Treating nuclei with
reagents known to alter
histone methylation or acetylation caused nuclei to grow less while still importing to a similar extent or to grow larger without significantly increasing import. Nuclear growth but not import was increased in live sea urchin embryos treated with the
DNA methylator
N-nitrosodimethylamine. These data suggest that nuclear import is not the primary driving force for nuclear growth. Instead, we observed that nuclear
blebs expanded preferentially at sites of high
chromatin density and
lamin addition, whereas small
Benzonase-treated nuclei lacking
DNA exhibited reduced
lamin incorporation into the nuclear envelope. In summary, we report experimental conditions where nuclear import is not sufficient to drive nuclear growth, hypothesizing that this uncoupling is a result of altered
chromatin structure. [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text].