A model for nucleocytoplasmic transport of ribonucleoprotein particles

Some aspects of the transport of ribonucleoprotein particles from nucleus to cytoplasm have been derived from in vitro assays employing isolated nuclei. The transport process has an apparent activation energy of 13 kcal/mol, shows an Arrhenius relationship without evidence of a transition between 35 and 0 °C, requires hydrolysis of one high-energy phosphate bond per nucleotide in transported RNA. All of these analyses of the process are independent of the type of ribonucleoprotein particle (ribosomal or messenger) transported. A serious conceptual difficulty arises when the size of the transported particles is considered. They must presumably travel through an aqueous channel in the nuclear envelope (since lipid phase transitions do not appear in the Arrhenius graphs), but all of the transported particles are too large to pass through the nuclear pore complexes. In reviewing what is known about ribonucleoprotein structure, we find common features which suggest the following model: 1. (1) the RNA chain is exposed at the surface of the particles 2. (2) small, local regions of particle structure "unfold" 3. (3) these unfolded (linear) segments of RNA interact with a translocation mechanism containing a nucleoside triphosphatase 4. (4) the RNA chain is linearly translocated through the pore channel the length of one nucleotide for each high-energy phosphate bond hydrolyzed 5. (5) the particle then refolds outside of the nuclear envelope.