About KPN / Karyopherin:
Karyopherins are proteins that help eukaryotic cells transport molecules between their cytoplasm and nucleus. The karyoplasm is the fluid that surrounds the nucleus (or nucleoplasm). In most cases, karyopherin-mediated transport occurs through nuclear pores, which serve as a portal into and out of the nucleus. To pass through the nuclear pore, most proteins need karyopherins.
Karyopherins Function
Karyopherins may act as imports (helping proteins enter the nucleus) or exports (helping proteins leave the nucleus) (i.e., helping proteins get out of the nucleus). The transporter classification database belongs to the nuclear pore complex family (TCDB). The Ran gradient provides energy for transportation.
KPN Structure
Several karyopherins are sequestered in stress granules, cytoplasmic aggregates of ribonucleoprotein complexes, as they avoid shuttling between the nucleus and the cytoplasm.
Karyopherins Mechanism
Importin beta is a form of karyopherin that helps cargo proteins get into the nucleus. First, it binds importin alpha, a form of karyopherin that binds the cargo protein in the cytoplasm, before the cargo protein is imported into the nucleus through the nuclear pore driven by energy from the Ran gradient. The cargo separates from the karyopherins once within the nucleus.
Without the aid of the importin alpha adapter protein, Importin beta can transport proteins into the nucleus.
Small RNAs are exported using Ran-GTP for directionality by karyopherin transport receptors, but mRNA is exported using a different mechanism that involves various quality controls. mRNA is exported in the form of very large mRNP complexes, which begin to form during RNA processing when the transcription export (TREX) complex binds to the mRNA. These mRNP complexes dock on the pore's inner surface, where they are subjected to quality control by the exosome and other surveillance activities.