© 1999 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999

8.5.4.2 Nuclear Pore Complexes

The most distinctive feature of the nuclear envelope is the presence of numerous nuclear pores (Fig. 8.46), small cylindrical channels with eightfold symmetry that extend through both membranes and provide direct contact between cytoplasm and nucleoplasm.^{1003,3403-3405} Each pore complex marks a point of fusion between the inner and outer membranes. Elements of the cytoskeleton appear to be attached to many pores, possibly allowing direct mechanical regulation of pore activity.^3406,3407

Each nuclear pore complex is a huge multimolecular assemblage measuring 70-90 nm in diameter, with a mass of 125 million daltons, ~34 times the size of a ribosome. Up to 100 different nucleoporin protein molecules make up the structure.¹⁰⁰⁴ Early experiments with passive gold particles showed that cytoplasmic particles with diameters of 5-6 nm passed into the nucleus in ~200 sec, those with diameters of 9-10 nm took ~10⁴ sec, but particles larger than 15 nm didn't seem to enter at all.⁹³⁹ Closer examination has revealed that the pores are actually large enough to allow the passage of substrates as large as 23-26 nm,^1003,1004 but this is still much too narrow for nanorobots or their flexible processes to pass through without damaging the mechanism. The nuclear localization sequence (NLS), a molecular tag consisting of 1-2 short sequences of amino acids, marks cytoplasmic proteins for active transport through the nuclear pores. Small (~40 nm) arm-like import receptors (cytoplasmic filaments) ringing the mouth of the pore bind to a protein cargo tagged with an NLS, then flex toward the pore to shove the cargo into the mouth.^{1264,3408,3409}

The density of pores across the surface of the nuclear envelope varies greatly, depending mainly on cell type and the amount of RNA being exported to the cytoplasm. Values range from 3-4 pores/micron² in some white cells up to 50 pores/micron² in oocytes and a theoretical maximum density of 60 pores/micron².⁹³⁹ A typical ~20 micron human cell has 2000-4000 pores embedded in its nuclear surface,¹⁰⁰⁴ a mean density of 10-20 pores/micron². Pore structures may protrude at most ~100 nm into the nucleoplasmic space.

Last updated on 20 February 2003