Nucleocytoplasmic exchange is mediated by nuclear pore complexes (NPCs). They're massive protein complexes that join the inner and outer nuclear membranes to create channels that span the nuclear envelope. Nucleoporins, which are made up of about 30 different protein components, assemble in many copies to form a complicated cylindrical architecture to the nuclear pores (30883195, 30943044).
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Hampoelz B, Andres-Pons A, Kastritis P, Beck M. Structure and Assembly of the Nuclear Pore Complex. Annu Rev Biophys. 2019 May 6;48:515-536. doi: 10.1146/annurev-biophys-052118-115308. Epub 2019 Apr 3. PMID: 30943044.