The transport of proteins across the nuclear envelope is a selective, multi-step
process involving several cytoplasmic factors. Proteins must be recognized as
import substrates, dock at the nuclear pore complex and translocate across
the nuclear envelope in an ATP-dependent fashion. Several cytosolic and
nuclear proteins that are central to this process have been identified. For
example, two cytosolic factors critically involved in the recognition and docking
process are the karyopherin α and karyopherin β proteins. The karyopherin
holoenzyme is a heterodimer of α and β subunits. The nuclear protein
B23 (also referred to as nucleophosmin) is involved in ribosomal assembly and
rRNA transport. B23 is an abundant protein that is highly phosphorylated by
Cdc2 kinase during mitosis.