By Levi Clancy for Student Reader on
Eukaryotic cells are defined as having a nucleus, thus distinguishing eukaryotes from prokaryotes.
The nucleus is an organelle containing the genome and complex machinery to control gene expression, and is separated from the cytoplasm by a double-membrane. From the nucleus, genetic information flows to the cytoplasm for utilization.
In contrast, while eukaryotes encase their genome in a double-membrane, prokaryotes allow their genome to float in the cytoplasm (sometimes localized to one part of the cell).
The nucleus is surrounded by a double-membrane (aka nuclear membrane, nuclear envelope and nucleolemma) and nuclear lamina (structural scaffolding).
This double-membrane keeps DNA and proteins that function in the nucleus segregated from the cytoplasm. Some argue that the nuclear envelope originated as an endosymbiont; others argue the nuclear envelope originated as an invagination of the cellular membrane. The nuclear membrane is contiguous with the endoplasmic reticulum membrane and is reinforced by a cytoskeletal scaffold called the nuclear lamina.
Also, the nuclear membrane is laden with huge protein complexes that create nuclear pores that control the passage of molecules in and out. The nucleus also contains specialized structures such as: the nucleolus, a large dense structure where rRNA is transcribed and processed and where ribosomes are assembled; the smaller cajal body, where snRNPs are assembled.
What Is the Nucleus For?
Prokaryotic mRNAs are polycistronic, meaning they encode multiple proteins and do not require a special structure to specify the correct reading frame. However, eukaryotic mRNAs are monocistronic, meaning they only encode a single protein and require a special 5’ cap structure to initiate translation.
In eukaryotes, the DNA is transcribed into precursor RNA; pre-mRNA is processed to form mRNA; and the finished mRNA proceeds to the cytoplasm for translation. Translation of unprocessed RNA is detrimental. The nucleus is essential for eukaryotes, separating this unprocessed RNA from the translation machinery in the cytoplasm.
The nuclear envelope consists of a lipid bilayer.
It is broken down and recreated during mitosis. Long and fibrous lamin proteins form a layer of structural support for the nuclear envelope. Lamin is phosphorylated in prometaphase, causing a conformational change and the loss of laminal structural properties. Without laminal support, the nuclear membrane breaks apart and absorbs into the smooth endoplasmic reticulum. The endoplasmic reticulum breaks apart, but is bound to the lamin via the inner nuclear membrane Lamin B Receptor; and the lamin binds to chromatin.
As anaphase ends, dephosphorylation of existing lamin begins. Once the genetic material has fully segregated at the completion of anaphase, production of new lamin is well underway. The new lamin drags tubes of smooth endoplasmic reticulum across the surface of the chromatin; these tubes flatten and merge, forming a solid nuclear membrane. The endoplasmic reticulum and lamin detach themselves from the chromatin. In the mature daughter cell, the lamina is a continuous layer that is bound to the inner membrane of the nuclear envelope by emerin proteins.
Nuclear Pore Complex
Nuclear proteins are translocated via nuclear pore complexes (NPCs) into the nucleus.
Most proteins have a diameter that is much smaller than the diameter of the central transporter of nuclear pore complexes (NPCs). Yet proteins >30 kDa cannot diffuse through a nuclear pore. In the NPC, there are FG-nucleoporins that line the channel. FG-nucleoporins have long stretches of hydrophilic amino acids as well as hydrophobic sections called FG repeats composed of phenylalanine (F) and glycine (G). The FG repeats associate with each other to form a molecular web through which ions, small proteins, small metabolites and proteins less than 30 kDa can diffuse through.
Transport is unidirectional because of rapid dissociation of importin. Ran-GTP is only hydrolysed to Ran-GDP in the cytoplasm, favoring release of Ran and binding of cargo. Nuclear transport factor 2 (NTF2) binds Ran-GDP and FG-nucleoporins to return Ran-GDP back to the nucleus.
Importins α and β form a heterodimer together.
Binds to the basic NLS (nuclear localization signal) of the cargo.
Has low affinity interactions with FG domains in the nuclear pore.
Upon reaching the nucleoplasm, importin binds tightly with Ran-GTP. The cargo is released.
The importin/Ran-GTP complex diffuses back into the cytoplasm.
Ran-GTP interacts with Ran-GAP while in the cytoplasm.
Ran-GTP/GAP interactions convert Ran-GTP to Ran-GDP and cause release of importin.
Ran-GDP diffuses back into the nucleus
In the nucleus, Ran-GDP encounters Ran guanine nucleotide exchange factor (Ran-GFP) which converts Ran-GDP→Ran-GTP.