The nuclear envelope, also known as the nuclear envelope, is a double-layered membrane located at the junction of the nucleus and cytoplasm of a eukaryote. The nuclear membrane is highly selective in the transportation of material inside and outside the nucleus, and controls the exchange and transportation of information and information between the material inside and outside the nucleus.
Figure 1. Human cell nucleus.
The nuclear membrane is composed of two inner and outer unit membranes, each with a thickness of about 6.5 nanometers, and the gap between the two membranes is about 10-30 nanometers. The gap between the two membranes is called the nuclear nucleus. The nuclear nucleus also contains Enzyme. Ribosomal particles are attached to the outer surface of the outer layer of the nuclear membrane. In some cells, the outer membrane is continuous with the rough endoplasmic reticulum membrane. Because the endoplasmic reticulum membrane and the plasma membrane are continuous, the nuclear membrane space seems to communicate with the extracellular space. On the inner surface of the inner layer of the nuclear membrane, there is a layer of a network composed of peptide substances, which is used to maintain the shape of the cell nucleus and attach chromatin fibers; during the mitosis process, it has a certain effect on the rupture and reconstruction of the nuclear membrane. There are many scattered holes in the nuclear membrane, called nuclear pores. Around the nuclear pores, the inner layer of the nuclear membrane is connected to the outer layer. A nuclear pore is a channel through which the nucleus and the cytoplasm exchange material. The nuclear membrane is not completely continuous. There are many parts of the inner and outer membranes connected to each other, forming nuclear pores through the nuclear membrane.
The nuclear membrane has a typical lipid bilayer structure, which is defined as the outer and inner membrane regions based on the distribution. There are small nuclear pores between the membranes. The stability of the nuclear membrane is maintained by the mesh structure of the two intermediate filaments: the internal network forms a nuclear layer on the inner membrane. A looser network is formed externally to provide external support.
The outer membrane is connected to a part of the endoplasmic reticulum, but the protein concentration in the nuclear membrane is higher than in the endoplasmic reticulum cavity. This basic structure of the nuclear membrane can vary from species to species.
The inner nuclear membrane surrounds the nucleus and is covered by the nuclear layer, and can be connected to the outer nuclear membrane through the nuclear pore complex. The nuclear layer is composed of an intermediate wire mesh, which can stabilize the nuclear membrane and participate in the process of chromatin function and the entire gene expression. Although the inner and outer nuclear membranes are connected to the endoplasmic reticulum, the proteins embedded in the membrane tend to remain on the original area rather than being dispersed throughout the continuum, suggesting that there may still be discontinuous boundaries on the membrane.
There are thousands of nuclear pores in the nuclear membrane. Each large nuclear pore complex is about 100nm long, while the channels in the nuclear membrane are about 40nm wide. The nuclear pores can connect the inner and outer nuclear membranes.
Figure 2. Nuclear pores crossing the nuclear envelope.
The special role of the nuclear membrane is to concentrate nuclear material in a region near the center of the cell. The regionalization of nuclear material is conducive to the realization of its function. The nuclear membrane is permeable to matter. Ions can penetrate the nuclear membrane, and relatively small molecules such as amino acids, sugars, protamines, histones, RNases and DNases can also pass through. However, macromolecules such as gamma globulin and albumin need to enter and exit the nucleus through the nuclear pore. The nuclear membrane is highly selective for the transportation of substances inside and outside the nucleus. The permeability of ions and small molecules is regulated by the nuclear membrane, while the permeability of large molecules of nucleic acids and proteins is controlled by the selective permeability of the nuclear pore complex. If the original proteins in the nucleus (such as histones, RNA polymerase, etc.) are injected into the cytoplasm, they can be concentrated in the nucleus; non-nuclear proteins with molecular weights exceeding 60,000 are difficult to enter the nucleus through the nuclear membrane. It is speculated that protein molecules in the nucleus may contain special signal peptides in the structure, so that they can selectively pass through the nuclear membrane and be concentrated in the nucleus. As for proteins and nucleoprotein particles whose volume is obviously larger than the effective pathway of the nuclear pore complex (for example, the molecular size of the ribosome subunit reaches 15 nanometers), it is assumed that the molecular conformation is passed from spherical to rod-shaped; The interaction of certain receptor molecules at the edge of the nuclear pore allows the pore diameter to be enlarged and passed.
Burke, Brian.; et al. Nuclei take a position: managing nuclear location. Developmental Cell. 2009, 17 (5): 587–597.