Nuclear Pore Complexes (NPC)

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.