Nuclear Pore Complex: Structure and Function
Nuclear Pore Complex: Structure
Each complex consists of the association of numerous proteins, known as nucleoporins, that presents an octamer provision. The pore complex can be distinguished:
- Eight column proteins, which form the side walls of the pore. Each is attached to the perinuclear space by anchoring proteins. In addition, there are eight radial proteins that project from the columns into the lumen, delimiting a central channel pore or opening.
- A cytoplasmic outer ring protein consisting of eight units and a domestic nuclear ring, also octamer. From each ring, fibrillar proteins and filaments converge on the nuclear side, forming a nuclear basket.
Small molecules pass through the nuclear pore by simple diffusion, but larger ones do so only if they present some specific amino acids, termed nuclear localization signals. In which case, they are recognized by specific carriers, importins and exportins, which drive them through the pore in an active transport process that consumes GTP.
Nucleoplasm and Nucleolus
- Nucleoplasm: It is the internal environment of the nucleus and consists of a colloidal dispersion in a gel composed of ions, nucleotides, RNA, and proteins related to the synthesis, packaging, and expression of nucleic acids. In the nucleoplasm, there also exists a fibrillar network, the nucleoskeleton, similar in structure and function to the cytoskeleton present in the cytosol.
- Nucleolus: It is a region inside the nucleus that is more or less spherical, not surrounded by any membrane system, and is formed by the association of DNA, RNA, and proteins. The nucleolus is the morphological expression of ribosomal RNA synthesis and the assembly of proteins to form ribosomal subunits. For this reason, there are three regions in the nucleolus:
- Fibrillar center: Corresponding to the strands of chromatin where rRNA genes that have not yet begun to transcribe are located.
- Dense fibrillar component: Corresponding to chromatin regions that have already begun to transcribe and begin to assemble with ribosomal proteins that come from the cytoplasm.
- Granular component: Corresponding to ribosomal subunits at different stages of maturation.
Chromatin Structure
It is observed in the interphase nucleus of eukaryotic cells as a fibrillar network with a high affinity for basic dyes.
Chromatin consists of DNA strands attached to proteins and small amounts of RNA.
The DNA that forms the chromatin is double-helical DNA and is associated with proteins that can be of two types:
- Nonhistone chromosomal proteins: They are mostly enzymes involved in the processes of transcription (RNA polymerase) and DNA replication (DNA polymerase).
- Histone or histone chromosomal proteins: They are basic proteins rich in amino acids such as arginine and lysine, with low molecular mass, which are associated with DNA by interaction between the positive charges of their basic amino acids and the negative DNA phosphate groups.
The main function of histones is to achieve DNA packaging inside the nucleus. There are five types of histones known as: H1, H2A, H2B, H3, and H4. Histones H2A, H2B, H3, and H4 group, forming an octamer (packages of eight molecules). They are assembled from a central tetramer of histones H3 and H4, to which two heterodimers of histones H2A and H2B are later added.
Nucleosome Organization
DNA in chromatin is packaged inside the cell nucleus, forming a complex and ordered structure, in which we distinguish the following levels of structural organization: Nucleosome fiber (“string of pearls”), which Kornberg (1947) concluded was formed by the periodic repetition of a structure he called a nucleosome. This structure is formed by the histone octamer and 1.65 turns of the double helix of DNA around it. When this nucleosome is associated with histone H1, a chromatosome is formed.