Export Proteins

Some proteins involved in the translocation of mRNA through the nuclear pore »

Adaptors as export signals:
Substances, like mRNA, that are transported from the nucleus to the cytoplasm, contain an export signal that serves as a label saying "I should be transported to the cytoplasm". The label is recognized by export receptors. It is probably not the mRNA itself, but rather proteins associated with the mRNA, which are recognized by the export receptors. For example, some common proteins associated with mRNA (like HnRNP A1 and HnRNP K) have been shown to contain export signals.

Also the proteins associated with the 5' end CAP of the mRNA (called Cap Binding Complex CBC) acts as export signal. They are not crucial for the transport of mRNA, but may play a role in the directional translocation since the 5' end of the mRNA is transported in the lead.

Export receptors:
Export receptors bind to the export signals and carry the cargo (the mRNA) to the other side of the nuclear pore complex. In the case of mRNA transport, an export receptor called CRM1 is believed to be important.

Directional transport:
In order for the export protein to bind to the export signal on the cargo, a protein called RanGTP is needed. The whole complex (the mRNA with the proteins acting as export signals, the export receptor and RanGTP) is transported together through the nuclear pore complex, from the nucleus to the cytoplasm. In the cytoplasm, the complex disassociates and in this process RanGTP is hydrolized to RanGDP. The receptors are then recycled to the nucleus.

mRNA may only be transported from the nucleus to the cytoplasm and not in the other direction. The assymetry of RanGTP/RanGDP is thought to be one important factor of this directional transport of mRNA.