Vesicles roar, and don't collapse

Kick out the reticulons to close the envelope The nuclear envelope reforms when tube-forming reticulons are ejected from chromatin-associated ER tubules, say Anderson and Hetzer. 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 fl atten 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 fl uorescent 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 fl attening 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 When RCC1's tail gets between it and chromatin, it gets a little help from its partner, Ran, to move the tail aside, according to Hao and Macara. RCC1 is a chromatin-binding protein essential for chromosome condensation, mitosis, and nuclear envelope assembly, and is also the only known exchange factor for the all-important Ran GTPase. RCC1 is shaped like a doughnut with a tail. Both parts of the protein have been implicated in chromatin-binding—removal of either portion weakens, but does not prevent, chromatin binding— which suggests a synergy between doughnut and tail, but the mechanistic details have been lacking. Binding of Ran to one side of the doughnut prompts the other side to bind chromatin, but because the doughnut is a rigid structure, Ran binding cannot simply induce an allosteric change in its shape. The authors thus looked …