Nuclear Pore Complexes (NPC)

Nuclear pores are the only direct pore connecting the nucleoplasmic and cytoplasmic spaces in the cell. They span both membranes forming the nuclear envelope and have both active and passive transport mechanisms.

Nuclear Pore Complexes

Nuclear pore complexes (NPC) are large, ~120 MDa pores that form the only known passageway across both bilayers that form the nuclear envelope.  Electron microscopy studies have revealed many structural features of the NPC which measures approximately 100 nm in diameter.  The pores consist of both cytoplasmic and nucleoplasmic ring structures connected by a spoke-like complex.  Filaments extend off both faces of the pore, with the nuclear side filaments terminating in a basket-like structure.  AFM images taken on the cytplasmic side of the nuclear envelope isolated from oocytes of Xenopus laevis reveal torus-like structures in the membrane. 

Apparent in the central region of many pores is a large mass.  The identity and function of this central mass or central granule has been the subject of considerable debate in the literature.  Its location is linked with calcium stores located in the cisternal region of the nuclear envelope.  Specific release of these calcium stores through activation of IP3 receptors located in the nuclear envelope leads to the emergence of the central mass in the NPC.  Some studies suggest that the central mass is an integral component of the NPC, while others suggest it is cargo caught in transport or an artifact of the sample preparation treatment. 

Recent studies from our laboratory suggest that the large mass observed may represent vault particles traversing the NPC.  Fluorescence resonance energy transfer (FRET) measurements revealed the co-localization of vault particles and NPCs at the nuclear envelope.  Moreover, the strength of the FRET interaction was dependent on cisternal calcium levels which have been shown to affect the displacement of the central mass.   

A more direct link can be provided by directly imaging the co-localization of vault particles and NPCs using immunofluorescence.  NSOM has the resolution necessary to spatially localize individual NPCs.  Moreover, because of the simultaneous collection of both topography and fluorescence information, NSOM can map the location of NPCs in the topography image while locating fluorescently labeled vaults in the optical image.  Distinct features in the NSOM topography image reflect individual NPCs in the nuclear membrane, albeit with lower resolution than that observed with AFM.  In the NSOM fluorescence image, punctuate features arise from the labeled vault particles.  The full-width-half-maximum of the smallest features in the fluorescence image are approximately 80 nm, illustrating the high spatial resolution. 

AFM Image of NPCs

AFM image of the cytoplasmic side the nuclear envelope from Xenopus oocytes. Each toroidal shape is an individual NPC in the nuclear envelop with a diameter of approximately 110 nm. A central mass is observed in the channel of some NPCs. The location of this mass appears to respond to nuclear envelope calcium levels.
AFM of NPCs

NPCs and Calcium

Calcium levels in the nuclear envelope can be modified by stimulating IP3 channels. AFM images before (left) and after (right) stimulating calcium release shows large changes in the location of the central mass in the NPC.
NPC and calcium

NSOM Images of NPCs

Overlay between the NSOM topography and fluorescence images. Since each image is collected simultaneously, this provides a direct mapping of the topography and fluorescence signals. The overlay shows that the fluorescence signals mapping the location of vault particles strongly correlates with topography changes that localize the NPCs in the nuclear envelope. This provides direct evidence that vaults not only localize to the nuclear envelope, but interact with NPCs.
NSOM of NPC

Selected Papers

Moore-Nichols, D.; Arnott, A.; Dunn, R. C., Regulation of nuclear pore complex conformation by IP3 receptor activation. Biophys J 2002, 83 (3), 1421-1428.

Erickson, E. S.; Mooren, O. L.; Moore-Nichols, D.; Dunn, R. C., Activation of ryanodine receptors in the nuclear envelope alters the conformation of the nuclear pore complex. Biophys Chem 2004, 112 (1), 1-7.

Mooren, O. L.; Erickson, E. S.; Moore-Nichols, D.; Dunn, R. C., Nuclear side conformational changes in the nuclear pore complex following calcium release from the nuclear membrane. Phys Biol 2004, 1 (1-2), 125-134.

Erickson, E. S.; Mooren, O. L.; Moore, D.; Krogmeier, J. R.; Dunn, R. C., The role of nuclear envelope calcium in modifying nuclear pore complex structure. Can J Physiol Pharm 2006, 84 (3-4), 309-318.

Dickenson, N. E.; Moore, D.; Suprenant, K. A.; Dunn, R. C., Vault ribonucleoprotein particles and the central mass of the nuclear pore complex. Photochem Photobiol 2007, 83 (3), 686-691.