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Nuclear Trafficking and Maxwell's Daemon

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Part of the TCSS Symposium

Eukaryotic cells are characterized by having a nucleus, in which the genetic material is stored, surrounded by a cytoplasm where proteins are synthesized. This arrangement poses a question of communication. The information stored in DNA in chromosomes in the nucleus is first transcribed into messenger RNA (mRNA) that is then transported out of the nucleus, through nuclear pores, to the cytoplasm where it is translated into proteins. Many of these proteins ultimately function in the nucleus and so have to be imported from the cytoplasm. The nucleus is separated from the cytoplasm by the nuclear membrane and transport between the two compartments is mediated by nuclear pores. The transport of large macromolecules, such as proteins and RNAs, though the nuclear pores is selective and requires metabolic energy. The nuclear pores themselves have a ~30 nm diameter central transport channel, but movement through this channel is restricted by its containing a high concentration of proteins (called “nucleoporins”) in which the chains lack regular structure. The high concentration of these unfolded protein chains restricts the conformations they can adopt and thus reduces their entropy, in a manner analogous to the way de Gennes proposed for the formation of polymer brushes. Inserting macromolecules into this gel of nucleoporins is inhibited because it increases the molecular crowding, further reducing their entropy. Specific transport molecules are used to overcome this inhibition by interacting with the nucleoporins in a way that releases sufficient free energy to overcome the loss of entropy. These transport factors do not, however, establish directionality of transport and simply diffuse back and forth through the pores with their bound cargo. Directionality is instead imposed by regulating the formation of the cargo:carrier complex in one compartment and its disassembly in the other compartment. Thus, energy is expended in sorting the material rather than actually transporting it through the pores, which happens simply by diffusion. Nuclear trafficking can therefore be thought of as a form of thermal ratchet in which Brownian motion is rectified. In nuclear protein import, for example, the energy is supplied by the hydroplysis of GTP . Thus, carriers of the karyophen family of transport factors bind their cargoes in the cytoplasm and diffuse back and forth through the pores. In the nucleus, the cargo:carrier complex is dissociated by the GTPase Ran in its GTP -bound form. The karyophenin:RanGTP complex then diffuses back to the cytoplasm where the GTP is hydrolyzed, leading to the dissociation of Ran and freeing the carrier for another transport cycle. The Ran is also imported back to the nucleus, where it is recharged with GTP .

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