The nuclear pore complex (NPC) is a ∼120 MD supramolecular machine implicated in nucleocytoplasmic transport, that is embedded in the double-membraned nuclear envelope (NE). The basic framework of the ∼120 nm diameter NPC consists of a 32 MD cytoplasmic ring, a 66 MD ‘plug-spoke’ assembly, and a 21 MD nuclear ring. The ‘central plug’ seen in en face views of the NPC reveals a rather variable appearance indicating that it is a dynamic structure. Projecting from the cytoplasmic ring are 8 short, twisted filaments (Fig. 1a), whereas the nuclear ring is topped with a ‘fishtrap’ made of 8 thin filaments that join distally to form a fragile, 30-50 nm distal diameter ring centered above the NPC proper (Fig. 1b). While the cytoplasmic filaments are sensitive to proteases, they as well as the nuclear fishtraps are resistant to RNase treatment. Removal of divalent cations destabilizes the distal rings and thereby opens the fishtraps, addition causes them to reform. Protruding from the tips of the radial spokes into perinuclear space are ‘knobs’ that might represent the large lumenal domain of gp210, a membrane-spanning glycoprotein (Fig. 1c) which, in turn, may play a topogenic role in membrane folding and/or act as a membrane-anchoring site for the NPC. The lectin wheat germ agglutinin (WGA) which is known to recognize the ‘nucleoporins’, a family of glycoproteins having O-linked N-acetyl-glucosamine, is found in two locations on the NPC (Fig. 1. d-f): (i) whereas the cytoplasmic filaments appear unlabelled (Fig. 1d&e), WGA-gold labels sites between the central plug and the cytoplasmic ring (Fig. le; i.e., at a radius of 25-35 nm), and (ii) it decorates the distal ring of the nuclear fishtraps (Fig. 1, d&f; arrowheads).
The human type VI collagen gene. mRNA and protein variants of the alpha 3 chain generated by alternative splicing of an additional 5-end exon.