NUCLEAR MEMBRANE: CHARACTERISTICS, FUNCTIONS AND COMPOSITION - BIOLOGY - 2025

The nuclear membrane, nuclear envelope or karyotheque, is a biological membrane, formed by a lipid bilayer that surrounds the genetic material of eukaryotic cells.

It is a fairly complex structure and equipped with a precise regulation system, made up of two bilayers: an internal and an external membrane. The space between the two membranes is called the perinuclear space, and it is approximately 20 to 40 nanometers wide.

Image via: unamenlinea.unam.mx

The outer membrane forms a continuum with the endoplasmic reticulum. For this reason, it has ribosomes anchored in its structure.

The membrane is characterized by the presence of nuclear pores that mediate the traffic of substances from the interior of the nucleus to the cytoplasm of the cell, and vice versa.

The passage of molecules between these two compartments is quite busy. RNA and ribosomal subunits must be constantly transferred from the nucleus to the cytoplasm, while histones, DNA, RNA polymerase and other substances necessary for the activity of the nucleus must be imported from the cytoplasm to the nucleus.

The nuclear membrane contains a significant number of proteins that are involved in the organization of chromatin and also in the regulation of genes.

General characteristics

Source Coutinho HD, Falcão-Silva VS, Fernandes Gonçalves G, Batista da Nóbrega R, via Wikimedia Commons

The nuclear membrane is one of the most prominent distinguishing features of eukaryotic cells. It is a highly organized double biological membrane, which encloses the nuclear genetic material of the cell - the nucleoplasm.

Inside we find chromatin, a substance made up of DNA bound to various proteins, mainly histones that allow its effective packaging. It is divided into euchromatin and heterochromatin.

Images obtained by electron microcopy reveal that the outer membrane forms a continuum with the endoplasmic reticulum, thus it also has ribosomes anchored to the membrane. Similarly, the perinuclear space forms a continuum with the lumen of the endoplasmic reticulum.

Anchored on the side of the nucleoplasm in the inner membrane, we find a sheet-like structure formed by protein filaments called “nuclear sheet”.

The membrane of the nucleus is perforated by a series of pores that allow the regulated traffic of substances between nuclear and cytoplasmic behaviors. In mammals, for example, it is estimated that there are an average of 3,000 to 4,000 pores.

There are very compact chromatin masses that are adhered to the inner membrane of the envelope, with the exception of areas where there are pores.

Function

The most intuitive function of the nuclear membrane is to maintain a separation between the nucleoplasm - the contents of the nucleus - and the cytoplasm of the cell.

In this way, DNA is kept safe and isolated from chemical reactions that take place in the cytoplasm and could affect genetic material in a negative way.

This barrier provides a physical separation to nuclear processes, such as transcription, and from cytoplasmic processes, such as translation.

The selective transport of macromolecules between the interior of the nucleus and the cytoplasm occurs thanks to the presence of nuclear pores, and they allow the regulation of gene expression. For example, in terms of the splicing of pre-messenger RNA and the degradation of mature messengers.

One of the key elements is the nuclear lamina. This helps support the nucleus, as well as providing an anchoring site for the chromatin fibers.

In conclusion, the core membrane is not a passive or static barrier. It contributes to the organization of chromatin, the expression of genes, the anchoring of the nucleus to the cytoskeleton, the processes of cell division, and possibly has other functions.

Training

During the processes of nucleus division, the formation of a new nuclear envelope is necessary, since, eventually, the membrane disappears.

This is formed from vesicular components from the rough endoplasmic reticulum. Microtubules and cellular motors of the cytoskeleton actively participate in this process.

Composition

The nuclear envelope is made up of two lipid bilayers made up of typical phospholipids, with several integral proteins. The space between the two membranes is called the intramembrane or perinuclear space, which continues with the lumen of the endoplasmic reticulum.

On the inner side of the inner nuclear membrane there is a distinctive layer made up of intermediate filaments, called the nuclear lamina, attached to the inner membrane proteins by means of heterochromarin H.

The nuclear envelope has numerous nuclear pores, which contain the nuclear pore complexes. These are cylinder-shaped structures made up of 30 nucleoporins (these will be described in depth later). With a central diameter of about 125 nanometers.

Nuclear membrane proteins

Despite the continuity with the reticulum, both the outer and inner membranes present a group of specific proteins that are not found in the endoplasmic reticulum. The most prominent are the following:

Nucleoporins

Among these specific proteins of the nuclear membrane we have the nucleoporins (also known in the literature as Nups). These form a structure called the nuclear pore complex, which consists of a series of aqueous channels that allow the bidirectional exchange of proteins, RNA and other molecules.

In other words, nucleoporins function as a kind of molecular "gate" that very selectively mediates the passage of various molecules.

The hydrophobic interior of the channel excludes certain macromolecules, depending on the size of the same and its level of polarity. Small molecules, approximately less than 40 kDa, or hydrophobic, can diffuse passively through the pore complex.

In contrast, molecules of a polar nature that are larger need a nuclear transporter to enter the nucleus.

Transport through the nuclear pore complex

Transportation through these complexes is quite effective. About 100 histone molecules can pass through a single pore per minute.

The protein that must be delivered to the nucleus must bind to importin alpha. Importin beta binds this complex to an outer ring. Thus, the protein-associated importin alpha manages to cross the pore complex. Finally, importin beta dissociates from the system in the cytoplasm and importin alpha dissociates already within the nucleus.

Inner membrane proteins

Another series of proteins are specific to the inner membrane. However, most of this group of almost 60 integral membrane proteins has not been characterized, although it has been established that they interact with the lamina and with chromatin.

There is increasing evidence that supports diverse and essential functions for the inner nuclear membrane. It appears to play a role in the organization of chromatin, in the expression of genes, and in the metabolism of genetic material.

In fact, it has been discovered that the wrong location and function of the proteins that make up the inner membrane are linked to a large number of diseases in humans.

Outer membrane proteins

The third class of specific nuclear membrane proteins reside in the outer portion of said structure. It is a very heterogeneous group of integral membrane proteins that share a common domain called KASH.

The proteins found in the outer region form a kind of "bridge" with the proteins of the inner nuclear membrane.

These physical connections between the cytoskeleton and chromatin appear to be relevant to the events of transcription, replication, and DNA repair mechanisms.

Foil proteins

The final group of nuclear membrane proteins is made up of the lamina proteins, a network of intermediate filaments that are composed of type A and B laminates. The lamina is 30 to 100 nanometers thick.

The lamina is a crucial structure that provides stability to the nucleus, particularly in tissues that are in constant exposure to mechanical forces, such as muscle tissues.

Similar to the internal proteins of the nuclear membrane, mutations in the lamina are closely related to a large number of very diverse human diseases.

In addition, more and more evidence is found linking the nuclear lamina with aging. All of this highlights the importance of nuclear membrane proteins in the overall functioning of the cell.

Nuclear membrane in plants

In the plant kingdom, the nuclear envelope is a very important membrane system, although it has been very little studied. Despite the fact that there is no exact knowledge of the proteins that make up the nuclear membrane in higher plants, certain differences with the rest of the kingdoms have been specified.

Plants do not have sequences homologous to the laminae and, instead of the centrosomes, it is the nuclear membrane that acts as the organizing center for microtubules.

For this reason, the study of the interactions of the nuclear envelope in plants with the elements of the cytoskeleton is a relevant subject of study.

References

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