| Abstract |
Eukaryotic cells are characterized by having a clearly defined nucleus that encloses genetic material, separating it from the rest of the cell. The regulation of protein partitioning between the nucleus and cytoplasm is crucial for controlling specific cellular functions; any imbalance in protein partitioning can lead to a range of diseases, highlighting its significance in cellular physiology. The major mechanism for establishing nuclear composition is the trafficking of proteins in and out of the nucleus by karyopherins after the formation of the nuclear envelope. Regulating protein partitioning is particularly challenging during early embryonic development due to the rapid and successive cell divisions that lead to a dramatic reduction in cell size. Further, the embryo is maternally deposited with vast quantities of proteins that must be appropriately incorporated into an exponentially increasing number of nuclei. How such large quantities of proteins are robustly partitioned into the nucleus during these dynamic developmental processes remains unclear. During my Ph.D., I studied the nuclear partitioning of the Xenopus laevis embryonic linker histone H1.8, a protein that alters the chromatin structure in a concentration-dependent manner. By using quantitative microscopy of Xenopus laevis egg extract, I provide evidence H1.8 is partitioned into the nucleus through a mechanism independent of nuclear import. H1.8 has already been shown to be one of the earliest proteins to partition within the nucleus during embryonic development. However, I demonstrate that the import kinetics of H1.8 is insufficient to account for its rapid nuclear partitioning. Further, I discovered that H1.8 is present in the nucleus and cytoplasm as liquid condensates. As the nucleus expands and grows in interphase, the nuclear condensates dissolve, suggesting that they act as reservoirs of proteins, potentially for DNA replication. The dissolution also suggests that most nuclear H1.8 is already present during the nuclear assembly, thus indicating that the partition of H1.8 inside the nucleus is not solely dependent on its import. Prior to the formation of the nucleus, I observe that the surface of the chromatid nucleates H1.8 as condensates similar to the formation of dew droplets on a cold surface. These condensates are then sequestered into the nucleus as the cell cycle progresses, leading to the protein reservoirs we observe in the nucleus. Such a mechanism allows for instantaneous enrichment of excess H1.8 inside the nucleus. Furthermore, I demonstrate that the cytoplasmic H1.8 condensates modulate the nucleation of H1.8 on the chromatid surface by buffering the soluble concentration of H1.8. This ensures that the amount of surface condensates is independent of the nucleus-to-cytoplasmic ratio, thus potentially allowing for a fixed enrichment of proteins despite the reduction in cell size during embryonic development. Such a mechanism would be crucial for key structural proteins, like H1.8, that maintain DNA packaging and structure in a concentration-dependent manner. Taken together, I propose that the nucleation of condensates on the surface of mitotic chromatids and subsequent wetting can provide an alternative mechanism to nuclear trafficking in regulating nuclear composition. |