Tubulohelical Membrane Arrays, Annulate Lamellae and Nuclear Pores: Tripartite Membrane Architecture with the Participation of Nucleoporins

The interest in nucleoporins originates from their identification as constituents of nuclear pores. The latter are visible as prominent annuli in the electron microscope (Callan et al., 1949). Comprehensive studies carried out in the 1950s led to these general conclusions: i) all nuclear envelope (NE) have pores, and ii) irrespective of species or cell type, the pore complexes show similarities in shape and size. This very much supported the hypotheses suggesting that these pores might act as transport channels between the nucleus and the cytoplasm of eukaryotic cells. A breakthrough for these ideas was achieved by the combination of microinjection techniques with transmission electron microscopy (TEM). In 1962, Feldherr injected contrasting permeants into the cytoplasm of amebas and observed their path through pores using TEM. Together with the observation of giant polycystronic messengers in transit through nuclear pores (Stevens and Swift, 1966), these experiments determined the direction of further study in the context of nucleocytoplasmic transport.