Notes on Nucleopore

Series of small perforations present on the double membrane covering the nucleus is known as the nuclear envelope. Its critical function is to separate the contents of the nucleus from cytoplasm in cells.

According to two scientists, Callen and Tomlin, 1950; nuclear envelopes contain small perforations referred to as nuclear pores. These pores are a large complex of proteins, the purpose of which is the transportation of molecules. On the margin of the pores the two membranes of the envelope become continuous. The pores are 400-1000 Angstrom in diameter and are circular and sometimes polygonal in shape. 

Detailed structure of nucleopores:

Apart from being a simple open channel, nucleopores also possess extra material which include blebs, diaphragms, septums, and microcylinders. Approximately 3000-4000 pores are present on the surface of a mammalian cell.

The nucleopores are highly selective, allowing only newly formed ribosome units to pass through, while restricting the active ribosome units from entering.

Large multiprotein complexes are present inside each nuclear pores known as the nuclear pore complex. The complex has an octagonal symmetry and has a cylindrical structure. At the margin of the nuclear pore complex, the inner and outer membrane fuse to form a continuous structure.

In general the pore complex consists of nine microcylinders, one wider central and eight peripheral.

The nucleopore has multiple copies of roughly 30 proteins that are different from each other. The specific proteins that interact among themselves to make the nuclear pore complex are known as nucleoporins. Alpha and beta solenoid proteins are found in 50% of the nucleoporins and the remaining half have intrinsically disordered proteins.

Uses of nucleopores:

• The main use of nucleopores is that they regulate the flow of substances in and out of the nucleus. The movement of the ions and molecules occur via simple diffusion, indicating a movement from a region of higher concentration to a region of lower concentration in an energetically favourable way.
• Nucleopores allow proteins with special sequence tags known as nuclear localization signals, to enter the nucleus from the cytoplasm.
• RNAs that have a nuclear export sequence tag and are transcribed in the nucleus are allowed to diffuse to the cytoplasm via the nucleopores.
• The nucleopore structure also gives rise to the nucleopore filters for research purposes.

Transport of molecules and ions using the nucleopore:

The nucleopores show the bi-directional movement of molecules between the cytoplasm and the nucleus. The proteins that are synthesised in the cytoplasm but function in the nucleus are imported to the nucleus via the nucleopores. Similarly the RNA and ribosomal subunits are exported in the cytoplasm.

The molecules and ions are transported by the nucleopore via two modes:

• Passive diffusion
• Active transport

Passive diffusion is seen in molecules with a size upto 50kDa, however they can be actively transported as well when needed.

Active transport in and out of the nucleus via nucleopores occurs for proteins that are larger than 50kDa.However, the proteins also contain certain specific amino acid sequences that function as nuclear localization signals. These are leucine rich sequences on a protein, directing the nuclear protein from the nucleus to the cytoplasm.

Whatman nuclepore track-etch membrane:

A nucleopore filter is one with holes of sizes of a few micrometres made in the polycarbonate membrane; the idea for the same was adopted from the actual structure of the nucleopore present on the envelope of the nucleus.

The filters are created using radiation. The plastic or polycarbonate membrane is exposed to radiation that causes the plastic membrane to weaken, creating holes in them.

The nucleopore filters are used in microbiology labs for the purpose of counting bacteria by fluorescence microscopy since the filter has a flat surface and the cells to be counted get trapped at the top, separating the fluids and smaller particles.

The filters are sold under the brand name Nucleopore from Whatman.

 Conclusion:

It can be concluded that nucleopores are of prime importance. Since the nuclear envelope separates the nucleus and the cytoplasm the transport of molecules and ions would become impossible. Roughly about 3000-4000 pores are present on the surface of a mammalian cell. The pores are 400-1000 Angstrom in diameter and are circular and sometimes polygonal in shape. 

Nucleopores are present in the membrane allowing molecules and ions to pass in a bidirectional manner.

Adopting the idea from the structure of the nucleopore, filters were also developed for the purpose of research.