From Single Complexes to Single Molecules Using Tapping Mode Scanning Force Microscopy and Capacitance-Based Motion Detection

Passive and facilitated transport in nuclear pore complexes is largely uncoupled

Nuclear pore complexes provide the sole gateway for the exchange of material between nucleus and cytoplasm of interphase eukaryotic cells. They support two modes of transport: passive diffusion of ions, metabolites, and intermediate-sized macromolecules and facilitated, receptor-mediated translocation of proteins, RNA, and ribonucleoprotein complexes. It is generally assumed that both modes of transport occur through a single diffusion channel located within the central pore of the nuclear pore complex. To test this hypothesis, we studied the mutual effects between transporting molecules utilizing either the same or different modes of translocation. We find that the two modes of transport do not interfere with each other, but molecules utilizing a particular mode of transport do hinder motion of others utilizing the same pathway. We therefore conclude that the two modes of transport are largely segregated.

Polyproline type II conformation in the C-terminal domain of the nuclear pore complex protein gp210

gp210 is a major constituent of the nuclear pore complex (NPC) with possible structural and regulatory roles. It interacts with components of the NPC via its C-terminal domain (CTD), which follows a transmembrane domain and a massive ( approximately 200 kDa) N-terminal region that resides in the lumen of the perinuclear space. Here, we report the solution structure of the human gp210 CTD as determined by various spectroscopic techniques. In water, the CTD adopts an extended, largely unordered conformation, which contains a significant amount of left-handed polyproline type II (PII) helical structure. The conformation of the CTD is altered by high pH, charged detergents, and the hydrogen bond-promoting reagent trifluoroethanol (TFE), which decrease the PII fraction of the fragment. TFE also induces a conformational change in a region containing an SPXX motif whose serine becomes specifically phosphorylated during mitosis. We propose that PII elements in the CTD may play a role in its interaction with the NPC and may serve as recognition sites for regulatory proteins bearing WW or other, unknown PII-binding motifs.