The Nuclear Transport Factor CSE1 Drives Macronuclear Volume Increase and Macronuclear Node Condensation In Stentor Coeruleus

The giant ciliate, Stentor coeruleus, provides a unique opportunity to study nuclear shape because its macronucleus undergoes a rapid, dramatic, and developmentally regulated shape change. During a 2 hour time period within cell division and regeneration, the 400 um long moniliform macronucleus condenses into a single mass, elongates into a vermiform shape, and then renodulates, returning to its original beads-on-a-string morphology.1Previous work from the 1960’s - 1980’s demonstrated that the macronuclear shape change is a highly regulated part of cell division and regeneration, but there were no molecular studies into this process.2,3 With the recent availability of a sequenced Stentor genome, a transcriptome during regeneration, and molecular tools like RNAi, it is now possible to investigate the molecular mechanisms that drive macronuclear shape change.4–6 We found that the volume of the macronucleus increases during condensation, suggesting an inflation-like mechanism. When the nuclear transport factor, CSE1, is knocked down by RNAi, this volume increase is reduced, and the nodes are unable to fuse. This leads to a long-lasting change in nuclear morphology. We found that CSE1 is mainly cytoplasmic during interphase and in early regeneration, and then accumulates inside the macronucleus during condensation. At the end of regeneration CSE1 is degraded while the macronucleus returns to its pre-condensation volume. We propose a model in which nuclear transport via CSE1 increases the volume of the macronucleus, driving the condensation of the many nodes into a single mass.