Question

Consider the events that lead to formation of the new nucleus at telophase. how do nuclear and cytosolic proteins become properly sorted so that the new nucleus contains nuclear proteins but not cytosolic proteins? how do you re-establish the ran-gtp concentration in the newly formed nucleus

Answer 1

The new nuclear envelope reassembles on the surface of the chromosomes, effectively preventing cytosolic proteins from being trapped between the chromosomes and the envelope. Cytosolic proteins are also excluded as clusters of chromosomes coalesce to form the complete nucleus. During this process nuclear pores are incorporated into the envelope. They permit selective import of nuclear proteins, causing the nucleus to expand while maintaining its characteristic protein composition. IMPORTANTLY, RanGEF is a protein that is bound to chromatin (review your Nuclear Transport notes). This means that RanGEF will travel with the chromosomes during mitosis and will be already INSIDE the newly formed nucleus in Telophase when the nuclear envelope surrounds the chromosomes. This allows the re-establishment of the Ran-GTP gradient across the nuclear envelope. RanGAP on the other hand, will be excluded from the newly formed nucleus.