Nuclear Pore: A Bidirectional Transport Machinery

Abstract

The human nuclear pore complex is a 120 nm structure composed of nearly 1000 protein molecules (multiple copies of nearly 30 different proteins) with a mass of 110–120 MDa. It spans the double membrane of the nuclear envelope and selectively transports both proteins, nucleic acids, and small signaling molecules bidirectionally. The diameter of the channel in the nuclear pore complex is approximately 5 nm in diameter and 45 nm in depth. Selective transport through the nuclear pore complex is mediated by nuclear transport receptors that bind to the cargo to be transported. Importins mediate transport of cargo molecules into the nucleus, whereas exportins facilitate the selective transport of cargo out of the nucleus. Cargoes with a nucleus localization signal are efficiently transported into the nucleus through the nuclear pore complex. The import and export cycles require GTP hydrolysis, and thus the transport process through the nuclear pore complex is energy-dependent. Since the nuclear pore complex is the gateway to the genome, the number of nuclear pore complexes varies during the different stages of the cell cycle. For example, between G1 and G2 phase of the cell cycle, the number of nuclear pore complexes at the nuclear envelope increase to accommodate greater transcriptional demand. Assembly of the nuclear pore complex like other cellular nanomachines is little understood.