Journey to the Center of the Cell

Abstract

![Figure][1] CREDIT: CHRIS BICKEL/ SCIENCE The animal cell nucleus houses the genetic material of the organism and therefore protects and maintains the blueprint for the cell and all its progeny. However, the nucleus is more than a simple repository for chromosomes. A dynamic organelle, the nucleus goes through astonishing transformations during each cell cycle, breaking down completely during mitosis and reforming afresh in each daughter cell after cell division. Within the nucleus, chromosomes are replicated and their DNA is transcribed to provide information that programs the physiology of the cell. Also, ribosomes assemble in the nucleus, then leave and carry out protein translation in the cytoplasm. All of this activity requires complex machineries that can respond to the changing needs of the cell throughout the cell cycle and may vary during development and by cell type. One of the defining features of the nucleus is its unique architecture. The nucleus is bounded by a nuclear envelope, a double layer of membranes punctuated by nuclear pores, which allow the passage of a huge variety of molecules into and out of the nucleus. The nuclear envelope is contiguous with the endoplasmic reticulum, a membranous labyrinth that provides the entry portal to the secretory pathway. The nuclear envelope is lined with a protein meshwork known as the nuclear lamina, which is composed of lamin proteins related to cytoplasmic intermediate filament proteins. Defects in the nuclear lamina have been linked to progeric diseases. In this issue, three Reviews examine various aspects of nuclear organization and dynamics. Trinkle-Mulcahy and Lamond (p. [1402][2]) describe how a combination of state-of-the-art proteomics and imaging technologies is contributing to a greater understanding of the dynamics of nuclear function in living cells in real time. Stewart et al. (p. 1408) describe how the nuclear envelope influences events within the nucleus and throughout the cell. Terry et al. (p. 1412) explain how the bidirectional transport of proteins and nucleic acids through nuclear pores can be regulated at many levels, from individual cargoes to global changes in nuclear pore transport characteristics. In a News story, Travis (p. [1400][3]) describes how a few researchers are making the case that proteins from the nucleus do double duty as components of a spindle matrix, a controversial structure alleged to help move chromosomes during cell division. Two Perspectives at Science 's Signal Transduction Knowledge Environment ( ) highlight mechanisms controlling nuclear transport. Alvarez-Gonzalez describes how poly(ADP-ribosyl)ation of p53 in response to DNA damage interferes with its nuclear export, and Swanson and Kopchick describe how the cytokine receptor GHR may traffic to the nucleus to regulate gene expression. In the future, it will be important to continue to advance our understanding of how the nucleus interacts with the rest of the cytoplasm throughout the cell cycle and how these interactions program organismal physiology and development. [1]: pending:yes [2]: /lookup/doi/10.1126/science.1142033 [3]: /lookup/doi/10.1126/science.318.5855.1400