Some proteins such as histones are relatively small and can pass through the 9nm pores easily. However, larger proteins, ribosomal subunits and mRNA molecules need another mechanism to pass through the nuclear envelope. This is done via active transport. Nuclear Localisation Sequences (NLS) on the cargo protein bind to a receptor protein called Importin to form an Importin-cargo complex. This complex is transported into the nucleus where the NLS releases the cargo protein and binds to a GTP binding protein called Ran. The resulting complex is transported out into the cytoplasm where the importin is recycled .
A proposed structure is that Nup54, Nup58 and Nup62 make up the central channel for the transport of molecules from the nucleusthrough to the cytoplasm. It is thought that Nup54 and Nup58 form a ringed structure with which Nup62 then associates. The structure of the ring is flexible and the Nup62 and Nup58 complex interact with both the cytoplasmic and nucleoplasmic sides of the nuclear envelope. The diameter of the Nup 54/Nup58 ring regulates transport activity of molecules through the nuclear pores .
- ↑ http://www.ks.uiuc.edu/Research/npc/
- ↑ Becker, Kleinsmith, Hardin (2007), The World of the Cell (Published by Pearson Education Inc.), Chapter 18, The Structural Basis of Cellular Information pg 545-546
- ↑ Solmaz SR, Chauhan R, Blobel G, Melčák I.. (2011). Molecular architecture of the transport channel of the nuclear pore complex.. Cell. 147 (3), 590-602