Nuclear envelope

Categories: Anatomy; cellular biology; physiology; transport mechanisms

The nuclear envelope is a double membrane system consisting of two concentric membranes. The membranes are separated by a fluid-filled space called the perinuclear cisterna, which measures about twenty to forty nanometers. Like other plant cell membranes, the nuclear envelope consists of two bilayers, both made of phospholipids, in which numerous proteins are embedded. Attachment sites for protein filaments are stitched on the innermost surface of the nuclear envelope. These protein filaments anchor the molecules of deoxyribonucleic acid (DNA) to the envelope and help keep them organized. The network of filaments that enmesh the nuclear envelope provides stability.

Large numbers of ribosomes are located on the outer surface of the envelope. The outermost membrane is continuous with the organelle called rough endoplasmic reticulum (RER) in the cytoplasm of the plant cell, which also has ribosomes attached to it. The space between the outer and inner membranes is also continuous with the rough endoplasmic reticulum space and can fill with newly synthesized proteins, just as the RER does. When the nuclear envelope breaks down during cell division, its fragments are similar to portions of the endoplasmic reticulum. This can be commonly observed at the root and shoot apices of a plant, where active mitosis (regular cell division for growth) occurs. Both bilayers of the envelope are fused at intervals to form many nuclear pores, which consist of protein complexes in clusters. The two membranes enclose a flattened sac and are connected at the nuclear pore sites.

Functions

The nuclear envelope surrounds the fluid portion of the nucleus, called the nucleoplasm, in all plant cells. The ribosomes on the outer surface of the envelope serve as sites for protein synthesis in addition to ribosomes located in the cytoplasm. The envelope is selectively permeable and therefore regulates the passage of materials and energy between the nucleoplasm and the cytoplasm. The envelope allows certain cell activities to be localized within the nucleus or outside in the cytoplasm. It also permits many different activities to go on simultaneously within and outside of the nucleus.

Like other membranes of the cell, the nuclear envelope membranes serve as important work surfaces for many chemical reactions in plants that are carried out by enzymes bound to the membranes. These functions are essential in the plant for the transport and use of minerals and water from the roots to cells of the stem and leaves. They are also important for the movement and use of carbohydrates, proteins, lipids, nucleic acids, and other chemical compounds in plant cells after photosynthesis, protein synthesis, and other biochemical activities have occurred.

Nuclear Pores

Nuclear pores, of about one hundred nanometers in diameter, perforate the nuclear envelope. These pores look like wheels with eight spokes when observed from the top. Each contains eight subunits over the region where the inner and outer membranes join. They form a ring of subunits that are fifteen to twenty nanometers in diameter. At the lip of each pore, the inner and outer membranes of the nuclear envelope are fused.

Each nuclear pore serves as a water-filled channel, and the arrangement allows transport in and out of the nucleus to occur in several ways. The nuclear pores allow the passage of materials to the cytoplasm from the interior of the nucleus and vice versa, but the process is highly selective, permitting only specific molecules to pass through these openings. An intricate protein structure called the pore complex lines each pore and regulates the entry and exit of certain large macromolecules and particles. Large molecules, including the subunits of ribosomes, cross the bilayers at the pores in highly controlled ways. Ions and small, water-soluble molecules cross the nuclear envelope at the pores.

Studies show that the pore can actually dilate more when it gets the appropriate signal. Studies have also shown that the signal is in the peptide sequences of the molecules. These signals are recognition sequences rich in the amino acids lysine, arginine, and proline. Nuclear pores in plants therefore exert control over the movement of materials. This is, for example, demonstrated in the fact that if a nucleus is extracted from a cell and placed into water, it swells; this can happen only if the pores prevent material from oozing out as the nucleus absorbs water.

Nuclear Lamina

The nuclear lamina is a layer of specific proteins, called lamins, attached to the inside membrane of the nuclear envelope. The layer consists of thin filaments (intermediate filaments) that are thirty to forty nanometers thick. Each filament is a polymer of lamin. There are two types of lamin: A-type lamins are inside, next to the nucleoplasm, and B-type lamins are near the inner part of the nuclear membrane. The nuclear lamina surrounds the nucleus, except at the nuclear pores. The lamina serves as a skeletal framework for the nucleus. It may be involved in the functional organization of the nucleus and may also play an important role in the breakdown and reassembly of the nuclear envelope during mitosis.

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