The Ins & Outs of Nucleocytoplasmic Transport: Studies of the Nuclear Pore Complex

Author(s): Tang, Jeffrey Hsin Nien | Advisor(s): Liphardt, Jan | Abstract: The nuclear pore complex (NPC) is one of the largest known protein structures in the cell. Evolutionarily conserved in eukaryotes ranging from fungi to plants and animals, the NPC is the main transporter of molecules between the cell cytoplasm and nucleus. Maintaining the proper compartment-specific localization of proteins and RNA is crucial for normal cell function, and the nuclear pore accomplishes this task both robustly and efficiently. Over the past several decades, insight into the composition, organization, structure, and mechanism of the NPC has been gradually teased out through careful experimentation. However, many questions about the pore's function remain unanswered. In this dissertation, I describe efforts aimed at elucidating several aspects of the NPC. First, I investigate the transport properties of the pore, specifically looking at how the nuclear transport receptor importin-β and the Ran GTPase interact not only with each other but also how they may affect the pore itself. The nucleoporin Nup153 is identified as an important player in the nuclear transport process which binds strongly to importin-β in a Ran-sensitive manner. Using multiple experimental techniques, the properties of importin-β, and Nup153's interactions are characterized and shown to be capable of modulating the selective permeability barrier of the NPC.Next, I examine how members of a major class of nuclear pore proteins, the scaffold nucleoporins, are both structurally and functionally similar to the karyopherin family of soluble nuclear transport receptors. Structures of the proteins Nup188 and Nup192 are analyzed and shown to resemble those of karyopherins. Furthermore, in vitro assays indicate that at least a subset of the scaffold nucleoporins behave functionally as transport receptors, hinting at an evolutionary relationship between these two important classes of proteins.Finally, a calcium-mediated phenomenon affecting the permeability of the NPC is explored. I show that certain cytosolic proteases are activated by millimolar concentrations of calcium ion which leads irreversibly to an increase in the nuclear pore's permeability to large molecules. A model for physiological pathways implicated in this effect is proposed.