Question

State what the function(s) of the following nuclear components and why each is important to the cell.
(Provide details, don’t just say “ribosomes are the location of translation.” What is translation) nuclear
envelope, nuclear lamin, ER, ribosomes, nuclear pore, and nucleolus. Remember, this is a study tool, the more detailed you are here, the better understanding you have of the info.

Answer 1

Nuclear Envelope

It is also called the Nuclear membrane. These membranes are usually found in eukaryotic cells and they are absent in prokaryotes. It is made up of two lipid bilayer membranes. There is an outer nuclear membrane and an inner nuclear membrane. There is a space between these membranes called the perinuclear space. The main function of this membrane is to protect the genetic material, which is the DNA.

The two membranes are connected to each other by nuclear pores. The nuclear envelope is supported by two sets of intermediate filaments. The internal support is given by the nuclear lamina and the external support is given by a loose network.

The outer membrane extends with the endoplasmic reticulum. That is they share the same common border. The outer membrane has a higher concentration of certain proteins like nesprin proteins,

The inner nuclear membrane covers the nucleoplasm. It is covered by the nuclear lamina.

Nuclear pores help in the movement of materials between the cytosol and nucleus. They link the inner and outer membrane.

Nuclear lamina

It is a dense network found inside the nucleus. Intermediate filaments and membrane-associated proteins are their compositions. The main function of the lamina is to provide mechanical support. Apart from this, they also regulate important cellular events like replication and cell division. It is associated with the inner nuclear membrane.

The nuclear lamina is composed of lamins and lamin associated proteins. Lamins are Type V intermediate filament. They are classified as either A-type(lamin A, C) or B-type ( lamin B1, B2). These polypeptides are mostly made up of alpha-helices, with multiple alpha helices interrupted by non-alpha-helical linkers. Both N-terminus and C-terminus are non-alpha-helical. The C terminus has a globular structure. These lamins are coded by three genes in vertebrates (Exception: Drosphila lamins are encoded by two genes and C elegans lamins are encoded by a single gene)

The lamin associated proteins are either integral or peripheral membrane proteins. Most important among them are:

• Lamina Associated Proteins 1 and 2 (LAP1 and LAP2)
• Emerin
• Lamin B-receptor(LBR)
• Otefin
• MAN1

Their main function is the attachment of nuclear lamina to the inner membrane.

The nuclear lamina is formed by the interaction between two lamin polypeptides. Here two alpha-helical regions are wound together to form a coiled structure. This is then followed by the head to tail association of the multiple dimers. These polymers then lay side-by-side, resulting in a 2D structure underlying the nuclear membrane.

Endoplasmic Reticulum

A continuous membrane system that extends from the nuclear membrane. The most important function of ER is protein synthesis. All eukaryotic cells have an ER. Depending on the morphology and function, they are of two types: Rough endoplasmic reticulum(RER) and Smooth endoplasmic reticulum(SER)

RER is called rough because of its rough appearance. This appearance is due to the attachment of ribosomes. The RER lies very close to the nuclear membrane and is an extension of the outer nuclear membrane. Ribosomes found on RER help in the synthesis of proteins. These synthesized proteins have different destinations. RER has several advantages because of its close proximity to the nucleus. Whenever there is an error in translation, it immediately sends a signal to the nucleus. There is another important mechanism in RER called Unfolded protein response. This signaling mechanism is activated when there is an accumulation of unfolded proteins in the ER lumen. This mechanism helps in reducing protein synthesis and increasing the protein folding capacity of the ER. If the mechanism fails, the cells undergo apoptosis.

Smooth Endoplasmic Reticulum is not associated with any ribosomes and hence they are smooth. SER is involved in the synthesis of lipids like cholesterol and phospholipids. It also several other functions. They are involved in the production of steroid hormones from cholesterol. In the liver, it helps in the detoxification of chemicals and drugs. There is a specialized SER called sarcoplasmic reticulum. It is found in the cytoplasm of striated muscles. It regulates calcium ion concentration in the cell.

Ribosome

A ribosome is the site of protein synthesis. They are found in both eukaryotes and prokaryotes. They are either seen free or are found in association with the rough endoplasmic reticulum.  They are highly abundant in cells.

Ribosomes are made up of proteins and ribosomal RNA(rRNA). (Prokaryotes: 40%- proteins and 60 %rRNA, Eukaryotes: 50% proteins and 50% rRNA). Ribosomes have two subunits. A larger subunit and a smaller subunit. These subunits are specified in terms of sedimentation rates, measured in Svedberg units (s). Prokaryotes: Smaller 30s and larger 50s subunit. Eukaryotes: smaller 40s and larger 60s subunit.

Ribosomes are the major site of translation. That is information carried by the mRNA gets converted into proteins with the help of anther RNA called tRNA. rRNA has a catalytic function. It helps in the formation of peptide bonds.

Nuclear pore

It is a protein lined channel in the nuclear membrane, which helps in the transport of molecules between the nucleus and cytoplasm. A nuclear pore is a large complex of proteins. They allow small ions or molecules to freely pass through them. Proteins having specific signals are recognized by these pores and allow their entry into the nucleus. The RNA that is synthesized has some export signals. This is recognized by the nuclear pore. The nuclear pore connects both the inner and outer membrane of the nucleus. There are approximately 1000 nuclear pore complexes in a vertebrate cell.

Proteins that make up the nuclear pores are called nucleoporins. Each pore will have around 456 protein molecules and 34 distinct nucleoporins. Half of the nucleoporins have solenoid protein domains and the other half has intrinsically disordered proteins.

Nucleolus

The nucleolus is a region inside the nucleus. It is composed of filamentous and granular materials.

Main functions:

• Synthesis and process of rRNA
• Association of rRNA with ribosomal proteins

Under an electron microscope, the nucleolus can be seen with two distinct areas, a darker fibrillar (pars fibrosa) and granulated (pars granulosa). These regions are interrupted by some light areas. The pars fibrosa contain heavily synthesized rRNA genes and rRNA. The pars granulosa contains pre-ribosomal particles. The light regions have nontranscribed DNA.

Malfunction of the nucleolus can cause serious issues like nucleolopathies. Their main components are of fibril center (FC), dense fibril center(DFC) and granular component(GC). rRNA transcription takes place in FC. An important protein in the rRNA transcription called fibrillarin is found in DFC. Another important protein involved in ribosome synthesis called nucleophosmin is found in GC.