A Structural Model for the Nuclear Pore Complex

Abstract

The nuclear pore complex (NPC) occupies a unique position in the architecture of the eucaryotic cell in that it forms the gateway between two environments, the nucleus and the cytoplasm, which are topologically equivalent but which differ in their macromolecular composition. Strictly speaking, the nucleus is not a membrane-bound organelle but rather can be thought of as an area of the cell surrounded by incompletely fused membrane vesicles with NPCs occupying the pores. Undoubtedly, one of the more fundamental functions of the NPC is simply to keep these pores open and to prevent the vesicles from fusing completely and isolating the nucleus from the cytoplasm. The existence of such pores in the nuclear envelope appears to be a simple way to allow the free passage of ions and small molecules between the nucleus and cytoplasm [14,15] without the necessity for a sophisticated system of channels which is required with a truly membrane bound organelle. Nevertheless, the NPC does carry out a barrier function since molecules larger than ~90A in diameter do not traverse it freely [14]. Large molecules are selectively imported into the nucleus and evidence is accumulating that such proteins contain a nuclear localization signal — a short sequence containing positively charged amino acid residues [3,5,10,16].