Importine

The term "importine" is used to describe import molecules into the nucleus.

In eukaryotic cells, DNA is isolated in its own organelle, the cell nucleus. This arrangement facilitates cellular regulation, especially gene expression and cell signalling, by controlling the passage of molecules to and from the nucleus. Molecules pass through the double-layer nuclear membrane via strictly controlled "holes" in the membrane known as nuclear pore complexes (NPCs). An NPC forms an aqueous channel that connects the nucleus to the cytoplasm and can be about 25 nm wide, large enough to allow large assemblies such as ribosomal subunits to pass through. A typical vertebrate cell has between 1000 and 10,000 NPCs. Each NPC contains 30-50 different proteins, called nucleoporins, which form a physical passage through the membrane and mediate the bidirectional transport of molecules to and from the nucleus.

Most small molecules such as ions and small proteins can pass the NPC by passive diffusion alone. However, larger molecules such as proteins, mRNA, tRNA and rRNA must first bind to carrier molecules (caryopherins or nuclear transport receptors) that specialize in transporting a specific charge through the NPC.

Karyopherins can be divided into importins (molecules imported into the nucleus) and exportins (molecules exported from the nucleus). However, some transport receptors act on both import and export molecules.

Once the nucleus exports mRNA to translate it in the cytoplasm, it must import back the proteins required for function, such as histones, ribosomal proteins required for ribosome assembly, signaling proteins such as transcription factors and splicing factors to process mRNA. Most large proteins are imported into the nucleus by importins, carrier molecules that recognize and transport cargo with a nuclear localization signal (NLS). Most importins are heterodimeric receptors consisting of two subunits:

Importin-alpha and importin-beta

Importin-Beta uses Importin-Alpha as an adapter molecule to recognize the nuclear localization signal in the cargo molecule. In other ways, however, importin beta can bind and transport the cargo itself.