There are roughly 3000–4000 nuclear pores located in the nuclear membrane of every eukaryotic cell. Each pore—pictured here using electron microscopy and 3D modeling—is composed of approximately 1000 proteins arranged into three layers of rings: a nuclear ring facing the nucleus; a cytoplasmic ring facing the cytoplasm; and an inner ring (pictured here in green and lime color) between the two. This three ring structure is called the nuclear pore complex. It acts as a gatekeeper that only allows proteins that are carrying the correct signal to pass into or out of the nucleus. But some viruses are able to mimic the appropriate signal and pass through undetected into the nucleus. Having the structure of the nuclear pore complex now in hand, scientists are studying how the passage through the pore works, how the cell completes the 3D puzzle of assembling the nuclear pore, and how the pores vary between different kinds of cells.
The image of the nuclear pore was produced using cryo-electron tomography and computer modeling. Tomography is a microscopy technique that uses electrons instead of light to take 3D images of the molecular world. Computer modeling is used to obtain atomic structures from such 3D images. The diameter of the nuclear pore is about 120 nm, which is 1,000 times less than the diameter of a human hair.
Jan Kosinski, PhD. and Martin Beck, PhD., Structural and Computational Biology Unit, EMBL, Heidelberg, Germany