Knockdown of reticulon (bottom) speeds formation of the nuclear envelope, indicated here by a smooth membrane around chromatin (red).
The nuclear envelope was once thought to disintegrate into vesicles during mitosis, but growing evidence suggests it is absorbed into the endoplasmic reticulum. Recent in vitro experiments by these authors have shown that, at the end of mitosis, ER tubules surround chromatin and gradually flatten out to form the nuclear envelope.
To test if the same process occurs in vivo, the authors tagged ER proteins, envelope proteins, and histones with fluorescent dyes. Chromatin remains free of ER membrane through metaphase, but by telophase, ER tubules started to attach to the chromatin, the authors showed. Once a few tubules were immobilized, more tubules slid into place alongside them, eventually coating the entire surface of the chromatin. As the nuclear envelope reformed, the ER tube-forming protein, reticulon, was cleared from the chromatin-associated membrane and collected in the surrounding ER tubules. The clearance of reticulon, which induces membrane curvature, coincided with flattening of the nuclear envelope.
From the onset of anaphase, complete closure of the NE took ∼10 min. Ejection of reticulons from the tubules was rate-limiting because overexpression delayed closure, and knockdown hastened it. “Formation of the nuclear envelope from tubular endoplasmic reticulum requires massive reorganization,” says PI Martin Hetzer, “so it's not too surprising that reticulons create a bottleneck as they are cleared.” These results appear to clinch the case for the ER as the source of the nuclear envelope. RR
