Nucleus Flashcards
What is the structure and function of a nuclear envelope?
- encloses DNA
- defines nuclear compartment
- nuclear pore complexes
- two membranes
Two concentric membranes
- > inner nuclear membrane: contains specific proteins that act as binding sites for chromatin and for the protein meshwork of the nuclear lamina that provides structural support
- > outer nuclear membrane: covere with ribosomes.
- > the proteins made on these ribosomes are transported into the space between the inner and outer nuclear membranes (the perinuclear space), which is continuous with the ERlumen
Bidirectional traffic
- > Into the nucleus: histones, DNA and RNA polymerases, gene regulatory proteins, RNA processing proteins
- > Into the cytosol: tRNAs and mRNAs
- > More complicated transport: ribosomal proteins are made in the cytosol, imported into the nucleus where they assemble with newly made ribosomal RNA into particles and are then exported again to the cytosol as part of a ribosomal subunit.
Describe the interconection of ER, nuclear membrane and nuclear pores.
The ER lumen it is continuous with the space between the two nuclear membranes. The lipid bilayers of the inner and outer nuclear membranes are connected at each nuclear pore.
Two networks of intermediate filaments provide mechanical support for the nuclear envelope; the intermediate filaments inside the nucleus form a special supporting structure called the nuclear lamina.
What is the structure of a nuclear pore?
More than 50 different proteins called nucleoporins , that are arranged to form an octagon.
A nuclear pore complex has four column subunits, which form the bulk of the pore wall:
-annular subunits
-lumenal subunits , which contain transmembrane proteins that anchor the complex to the nuclear membrane
-ring subunits, which form the cytosolic and nuclear faces
of the complex.
-fibrils protrude from both the cytosolic and the nuclear sides of the complex (forming nuclear basket on the inside and fibrillar protrusions on the outer side)
Each pore complex contains one or more open aqueous channels through which small water soluble molecules can passively diffuse.
Ribosomes and many other protein and RNA molecules bind to specific receptor proteins that ferry them actively through nuclear pore complexes.
-lysine rich sequence as a nuclear localization sequence (NLS)
What are nuclear localizing signals and nuclear import receptors?
Nuclear import receptors bind both to nucleoporins and to a NLS on the cargo proteins. Some proteins require an adaptor protein. nuclear import receptor. The adaptors are structurally related to nuclear import receptors and recognize NLS on cargo proteins. They also contain a NLS that binds them to an import receptor.
- soluble cytosolic proteins
- phenylalanine and glycine repeats (FG repeats) of porine fibrils serve as binding sites for the import receptors
What are nuclear export signals and nuclear export receptors?
Nuclear export receptors are structurally related to nuclear import receptors, and they are encoded by the same gene family of nuclear transport receptors, or karyopherins.
Works in a same way as NLS and nuclear import receptors but in an opposite way.
How is the nuclear transport driven?
The Ran GTPase drives directional transport through nuclear pore complexes. Ran is found in both the cytosol and the nucleus, and it is required for both the nuclear import and export systems.
- > Cytosol: Ran-GAP (Ran GTPase activating protein) + Ran-GDP
- > Nucleus: Ran-GEF (Ran guanine nucleotide exchange factor) bound to chromatine + Ran-GTP
Nuclear transport receptors are composed of repeated a helical motifs that stack into either large arches or snail shaped coils , depending on the particular receptor or adaptor. Cargo proteins and Ran GTP bind its different regions at the inside faces of the arches.
A nuclear import receptor bound to Ran GTP, a conserved
loop of the receptor becomes covered by bound Ran GTP, which, in the Ran free state of the receptor, is thought to be important for signal sequence binding.
How is the nuclear envelope broken down and reformed?
Proteins of the inner nuclear membrane are phosphorylated, and the nuclear pore complexes disassemble and disperse in the cytosol. The
phosphorylation of the lamins is thought to trigger the disassembly of the nuclear lamina , which in turn causes the nuclear envelope to break up. Dephosphorylation of the lamins is thought to help to reverse the process.
