Contribution of Electron Microscopy to the Study of the Nuclear Pore Complex Structure, Composition, and Function

In interphase eukaryotic cells, the nucleus is separated from the cytoplasm by a double nuclear membrane system called the nuclear envelope. The nuclear envelope enables the two cell compartments to have their distinct composition, and separates the genetic machinery from protein synthesis. Molecular exchange between the nucleus and the cytoplasm, however, is essential for keeping eukaryotic cells alive, and is involved in the regulation of many cellular functions, including gene expression. Thus, eukaryotic cells have evolved an elaborate system for maintaining constant communication between the nucleus and the cytoplasm. A major component of this system is the nuclear pore complex, a large and extremely elaborate protein structure embedded in the nuclear envelope which mediates bidirectional nucleocytoplasmic trafficking of macromolecules. The nuclear pore complex has a molecular mass of 125 megadaltons, and is composed of 30 different proteins, arranged in an octagonal structure that is 120 nm in diameter. The structure of the nuclear pore complex has been extensively investigated using a variety of electron microscopy techniques, including classical electron microscopy techniques such as embedding and thinsection, as well as state-of-the-art electron microscopic technologies such as cryo-tomography. In addition, immuno-gold electron microscopy has been used to reveal the molecular composition of the nuclear pore complex, and to dissect nuclear transport into distinct steps. This chapter reviews the contribution of several electron microscopy techniques to the study of the nuclear pore complex structure, composition and function.