Nuclear Pore

NUCLEAR PORE - Perforations with purpose
The word 'pore' is derived from the Greek 'poros' which translates to 'passage'. Neither 'passage' nor 'perforation' gives an indication of the complexity of the structure and functioning of a nuclear pore or 'nuclear pore complex'.
Each nuclear pore is about thirty times the mass of a ribosome and there are about 3000 to 4000 of them perforating the nuclear envelope of each cell.

Nuclear Pore - an operating analogy
The pore operates rather like a turnstile or ticket gate. Those entering the event area will need a ticket to operate the stile or gate. Small items can be passed through the turnstile but people with large items need special facilities.

The turnstile therefore not only controls the flow but is also selective. The ticket operating the turnstile, like certain proteins entering a nuclear pore, carries important destination information. It might be something like "Porham Stadium, Stand 4, Row E, Seat 32". Without this detailed information access is denied. In cell biology terms this entry information consists of a short protein sequence called a 'nuclear localisation signal'.

Some viruses use 'counterfeit tickets' to gain access
Just as some people try to use counterfeit tickets for a football match, some viruses use a counterfeit nuclear localisation signal to gain access to the nucleus.

Why is a nuclear pore complex needed?
In the item about the nuclear envelope we wrote of the need to keep the chromosomes and chemical reactions taking place in the nucleus separate from reactions taking place in the cytoplasm. This is true, but for the cell to function properly certain chemicals need to flow between organelles. This is called nucleocytoplasmic transport.The nucleus requires nucleotide triphosphates, proteins and various water soluble ions to make different types of RNA. These are also needed to make ribosome sub-units for export to the cytoplasm and to enable the cell to divide when needed.
Nuclear pore complexes control the flow into and out of the nucleus and check the credentials of all large molecules attempting to pass through.

Imports to the nucleus
The pore complex is constructed from more than one hundred different proteins and is a watery channel that can be as small as 9 nm in diameter.

Small water-soluble molecules and proteins of small molecular weight can pass through the pore unchallenged and unaccompanied.
The approach of a large molecule in the cytoplasm is detected by protein filaments that extend from the circumference of the octagonally shaped pore. If the approaching protein has a 'ticket' bearing the appropriate nuclear localisation signal it becomes attached to a nuclear import/export receptor or 'protein escort'. This molecular 'protein escort' guides the molecule through the pore 'turnstile' which can be made to accommodate large molecules. After assisting a protein through the nuclear pore 'turnstile' to the nucleus, the escort molecule links to and escorts protein molecules leaving the nucleus for the cytoplasm. The protein escort molecules or nuclear import/export receptors, are thought to shuttle between the cytoplasm and nucleus and back again.
Proteins permitted to enter the watery channel can do so in an unfolded state.

Export from the nucleus
The nucleus contains the operating instructions for the cell. For the instructions to be carried out, copies must leave the nucleus. They do this in the form of a ribonucleo protein complex in which RNA is coupled to protein including a 'nuclear export signal' protein. Ribosome construction sub-units are also exported from the nucleus.
Not a great deal is known about how molecules are exported from the nucleus to cytoplasm. It is known that on the nucleus side of the pore, protein filaments protrude in the shape of a cone shaped basket. It is thought that this structure guides molecules for export to the pore and checks them for an export signal. This happens before full admission into the pore is gained for controlled transport through the pore into the cytoplasm.

Big lay-off at cell division
By any standards a nuclear pore is a piece of really smart biological engineering. It is surprising therefore that when a cell divides, nuclear pores, like the nuclear envelope, are disassembled. They are re-assembled after the nucleus has divided and when the nuclear envelope is being rebuilt. This process means that between 3000 and 4000 nuclear pores are 'laid-off' until the nuclear envelope re-build takes place!
It is not clear how nuclear pores are disassembled. Are they stripped down to basic amino acid building blocks for example, or are they re-cycled as 'used units'? If so, how and where are they stored?

SUMMARY

• A nuclear pore is a sophisticated biological entry and exit control system guarding the contents and work of the 'Head Office' of the cell, the nucleus.
• The pore, working like a turnstile or ticket gate scrutinises large molecules attempting to enter or leave the nucleus whilst permitting small molecules to flow in and out freely.
• Only those large molecules carrying authorised 'address' information are allowed to enter the nuclear pore.
• Molecules accepted for entry are escorted through the nuclear pore or turnstile by a protein escort.
• Large molecules due to leave the nucleus are security checked in a similar way.
• When cell division takes place the entry/exit control system is disassembled with the nuclear envelope for the duration of the 're-build'.