Binding within the Nuclear Annuli and Its Possible Effect on Nucleocytoplasmic Exchanges

Evidence exists which indicates that the pores of the nuclear envelope are important pathways for macromolecular exchanges between the nucleoplasm and cytoplasm (1-4). The pores as seen with the electron microscope often contain an electron-opaque material (5-7), and it has been suggested that the presence of such pore material could affect nucleocytoplasmic exchanges (8). Investigations in which the passage of colloidal particles through the nuclear annuli of intact amebas was studied support this hypothesis, and further suggest that the colloidal particles bind to the material within the pores (3). If such binding does occur, it could be an important factor in controlling macromolecular exchanges between the nucleus and cytoplasm. To determine the extent to which binding to the pore material can occur, experiments were conducted in which isolated nuclei or isolated nuclear envelopes were treated with protected colloidal gold particles and subsequently examined with the electron microscope.