Name: nuclear transport controls protein entry
Uploaded: 2023-04-30T09:41:31.000Z
Duration: 1 min 45 s
Channel: JoVE

Nuclear transport is controlled by masking nuclear localization sequences, altering the pore size of the nuclear pore complexes, and secreting precursor proteins.
NLS is masked either by phosphorylation of cargo as observed in the viral transcriptional factor, v-Jun, or by inhibitory proteins that bind the NLS on the cargo as observed in the regulation of NF-kappaB by the I-kappaB. These mechanisms interfere with the NLS recognition process, thereby retaining the cargo in the cytosol or other organelles.
Nuclear pore size is modulated by cytoskeletal proteins to control the size of macromolecules passing through the NPC. As a tight aqueous pore, the NPC expands and contracts to allow selective molecules to squeeze through.
Regulation by precursor proteins is observed in cholesterol metabolism. Sterol response element-binding protein or SREBP, is a precursor of a transcription factor that is present as a transmembrane protein in the ER attached to the SREBP cleavage activation protein or SCAP that is bound to cholesterol.
At low cholesterol concentrations, SCAP changes conformation and the SREBP-SCAP complex gets transported to the Golgi apparatus.
Golgi-resident proteases cleave SR

regulation of nuclear protein sorting

Regulation of Nuclear Protein Sorting

Nuclear protein sorting regulates nucleus composition and gene expression, crucial for determining the fate of a eukaryotic cell. Hence, the entry and exit of molecules across the nuclear envelope is a tightly controlled process. Nuclear protein sorting can be inhibited by one of the following ways: 1) masking cargo signal sequences, 2) modifying the nuclear receptor&#39;s affinity for cargo, 3) controlling the nuclear pore size, 4) retaining the cargo during its transit to the cytosol or the nucleus and, 5) secreting precursor proteins.
Masking of sorting signals
Nuclear transport is initiated as nuclear receptors bind to the nuclear localization signals (NLS) or nuclear export signals (NES). To limit the accessibility of the cargo to the nuclear receptors, NLS or NES can be masked by two types of mechanisms. In the first mechanism, the cargo protein undergoes conformational changes through disulfide bond formation or phosphorylation so that it no longer fits into the binding site of the receptor. In the second process, the cargo may bind another molecule, such as DNA, mRNA, or a protein, that interferes with its binding to the receptor.
Modifying nuclear receptors
Phosphorylation of importin alpha by casein kinase II upon binding to NLS of a cargo enhances its importin beta binding affinity and nuclear import. Alternatively, increased levels of importin beta can promote enhanced trafficking of its substrate by outcompeting other nuclear receptors for binding sites on the Nuclear Pore Complexes or NPCs.
Regulating Nuclear pore size
Nuclear pore size also controls the flux of protein transport. Cytoskeletal proteins help constrict or dilate the nuclear pore diameter to restrict cargo entering the nucleus. Alternatively, the extracellular matrix can stretch the nuclear envelope and expand the nuclear pores for cargo import.
Cargo retention
Nuclear trafficking is also restricted when cargo binds to cytosolic factors such as 14-3-3 protein or is tagged for degradation, inhibiting unnecessary transport across the nucleus.
Precursor secretion
Some proteins such as p105 are secreted in the cytoplasm as inactive precursors. Phosphorylation of p105 signals its cleavage to release an active P50 that can be transported&#160;to the nucleus.

Nuclear protein sorting regulates nucleus composition and gene expression, crucial for determining the fate of a eukaryotic cell. Hence, the entry and ...

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JoVE Core

Cell Biology

Unit 1

Intracellular Compartments and Protein Sorting

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KEY TERMS AND CONCEPTS

nuclear transport controls protein entry

Nuclear transport controls which proteins enter and leave the nucleus, and it helps shape gene expression in eukaryotic cells. Because the nucleus must keep a specific composition, movement across the nuclear envelope is tightly regulated. Cells can slow or stop this transport in several ways.

One common control is to hide the protein’s sorting signal. Nuclear receptors first bind nuclear localization signals, or NLS, and nuclear export signals, or NES. A cargo protein can change shape through disulfide bond formation or phosphorylation, which prevents receptor binding. Cargo can also bind DNA, mRNA, or another protein, which blocks access to the signal.

Cells can also change the nuclear receptors themselves. When casein kinase II phosphorylates importin alpha after it binds an NLS-containing cargo, importin alpha binds importin beta more strongly and nuclear import increases. Higher levels of importin beta can also boost transport by competing with other nuclear receptors for binding sites at nuclear pore complexes, or NPCs.

The size of the nuclear pore also affects transport. Cytoskeletal proteins can constrict or widen the pore diameter and limit which cargo enters the nucleus. In another case, the extracellular matrix can stretch the nuclear envelope and enlarge the nuclear pores to support cargo import.

Cargo can be held back before it reaches the nucleus or cytosol. Some cargo binds cytosolic factors such as 14-3-3 protein, and some cargo is tagged for degradation, which reduces unnecessary transport. Other proteins are made as inactive precursors in the cytoplasm.

One example is p105, which remains as a precursor until phosphorylation triggers cleavage to form active p50. After this processing step, p50 can move to the nucleus. This shows how precursor secretion can control when a protein is ready for nuclear transport.

Nuclear Transport Controls Protein Entry

overview of protein sorting and transport

overview-of-protein-sorting-and-transport

Protein Targeting Inside Eukaryotic Cells

Protein targeting helps eukaryotic cells send proteins to the right organelle. Each membrane-bound organelle has its own protein needs, so proteins must be sorted after they are made. This process is called protein sorting. Protein sorting happens in two main ways: signal-based sorting and vesicle-based trafficking. Signal-based sorting uses sorting signals, which are specific amino acid sequences. These signals direct proteins to the correct place in the cell through gated transport or...

signal sequences and sorting receptors

Protein Targeting Signals and Sorting Receptors

Protein targeting signals help newly made proteins reach the correct place inside a cell. These short amino acid sequences act like address labels. They guide proteins to the organelle where they belong. Classical signal sequences are usually 15 to 60 amino acids long. They are found at the N-terminus, or beginning, of a polypeptide chain. These signals often include a basic region near the start, a hydrophobic core, and a polar C-terminus. The C-terminus also contains a signal cleavage site...

nuclear protein sorting

How Proteins Cross the Nuclear Envelope

Nuclear protein sorting controls how proteins, RNA, and ribosomes move between the nucleus and the cytosol. Histones, polymerases, and gene regulatory proteins are carried into the nucleus. RNA and ribosomes are exported to the cytosol. This selective transport helps regulate gene expression inside the cell. Proteins that enter the nucleus usually carry a nuclear localization signal, or NLS. Import receptors in the cytosol recognize this signal and bind the cargo. Proteins that leave the...

nuclear localization signals and import

Nuclear Import Signals in Proteins

Proteins that need to enter the nucleus carry a nuclear localization signal, or NLS. This short amino acid stretch helps the cell sort proteins to the right place. In many proteins, the signal is made of positively charged amino acids such as lysine and arginine and can appear as a loop or a signal patch. Classical nuclear localization signals have two main forms. A monopartite cNLS is a single cluster of 4 to 8 amino acids. A bipartite cNLS has two clusters of 2 to 3 amino acids with a...

nuclear export

Nuclear Export Signals and Cargo

Nuclear export signals help proteins and RNA leave the nucleus and reach the right place in the cell. The nucleus also keeps some proteins inside by using a nuclear retention sequence, or NRS, which anchors them to the nuclear lamins and blocks movement to the cytosol. Proteins that are not restricted can move after synthesis to the cytosol or to other organelles. They do this with the help of nuclear export signals, or NES. The transcript describes three NES types: the canonical leucine-rich...

directionality of nuclear transport

Ran GTP Gradient in Nuclear Transport

Ran GTP helps control the direction of nuclear transport in cells. Ran, or Ras-related nuclear protein, is a small G protein that switches between GTP-bound and GDP-bound forms. Its activity depends on two key regulators: RanGAP in the cytosol and RanGEF inside the nucleus. A higher RanGTP level builds up inside the nucleus because GTP is abundant in cells and Ran regulators are unevenly distributed. This RanGTP gradient across the nuclear envelope supports one-way movement of nuclear proteins.

Nuclear transport controls which proteins enter and leave the nucleus, and it helps shape gene expression in eukaryotic cells. Because the nucleus must keep a specific composition, movement across the nuclear envelope is tightly regulated. Cells can slow or stop this transport in several ways. One common control is to hide the protein’s sorting signal. Nuclear receptors first bind nuclear localization signals, or NLS, and nuclear export signals, or NES. A cargo protein can change shape through...

mitochondrial protein sorting

Mitochondrial Protein Import Routes

Mitochondrial protein import routes move proteins to the right place inside mitochondria. Mitochondria are double-membrane organelles in eukaryotes. They support cellular metabolism, signaling, ATP synthesis, and programmed cell death. Each of these jobs depends on proteins and enzymes reaching the correct mitochondrial subcompartment. Most mitochondrial proteins are made in the nucleus and then imported into the organelle as unfolded or loosely folded precursors. These precursors usually...

mitochondrial precursor proteins

How Mitochondria Recognize Protein Precursors

Mitochondria recognize precursor proteins before they enter the organelle. These precursors are newly made polypeptide chains that stay partially unfolded or loosely folded. Cytosolic chaperones, including heat shock protein 70 (Hsp70), and mitochondrial import stimulation factors (MSFs) help keep them from folding too soon. Precursors bound to MSFs are directed to the TOM70-TOM37 receptor complex. Precursors bound to Hsp70 chaperones are targeted to the TOM20-TOM22 receptor complex. Most...

translocation of proteins into the mitochondria

translocation-of-proteins-into-the-mitochondria

Mitochondrial Protein Sorting Pathways

Mitochondrial protein sorting pathways move precursor proteins to the outer membrane, intermembrane space, inner membrane, or matrix. These proteins use different translocases, which are protein machines that guide them to the right mitochondrial compartment. Outer membrane proteins follow two routes. Beta-barrel porins pass through the TOM complex and are inserted into the outer membrane by the SAM complex. Alpha-helical, membrane-anchored proteins also pass through TOM, but they are inserted...

energy to drive translocation

Mitochondrial Protein Import Energy

Mitochondrial protein import uses ATP hydrolysis and an electrochemical potential across the inner membrane. Newly made precursor proteins first bind to Hsp70 chaperones in the cytosol. These chaperones help guide the proteins to import receptors on the mitochondrial surface. ATP hydrolysis gives Hsp70 the energy to pass precursors to the TOM receptors on the outer membrane. TOM and TIM are the main import complexes that move proteins across the two mitochondrial membranes. Before transport,...

structure of porins

Porin Assembly in Mitochondria

Porin assembly in mitochondria depends on beta-barrel proteins that move across the outer membrane and fold into their final form. Mitochondria, chloroplasts, and gram-negative bacteria use these transmembrane proteins to let ions and metabolites diffuse freely across membranes. Mitochondrial porin precursors carry conserved beta signals at their C-terminal end. These beta signals contain the PoXGXXHyXHy motif, where Po is polar, X is any amino acid, G is glycine, and Hy is a large hydrophobic...

porin insertion in the outer mitochondrial membrane

porin-insertion-in-the-outer-mitochondrial-membrane

SAM Complex Assembly of Mitochondrial Porins

Mitochondrial porins are beta-barrel proteins that are built into the outer membrane with help from the TOM and SAM systems. Porin precursors first move through the TOM complex into the intermembrane space. They then bind TIM chaperones, which guide them to the Sorting and Assembly Machinery, or SAM, complex on the outer mitochondrial membrane. The SAM complex helps fold and place porins into the membrane, but the exact assembly route is still modeled in different ways. In one model, the porin...

protein transport into the inner mitochondrial membrane

protein-transport-into-the-inner-mitochondrial-membrane

Mitochondrial Import Pathways to the Inner Membrane

Mitochondrial import pathways move nuclear-encoded precursor proteins to the inner membrane in several steps. Two main translocons, TIM22 and TIM23, handle much of this sorting. A third translocase, OXA, helps insert some proteins into the membrane after they have moved partway through the system. TIM23 is a cation-selective pore, which means it favors positively charged signals. It stays closed by the N-terminal segment of the protein. Negative charges on TIM23 act like a receptor for the...

protein transport to the thylakoids

Thylakoid Protein Targeting Pathways

Thylakoid protein targeting pathways move precursor proteins to the chloroplast thylakoids, where photosynthesis takes place. Thylakoids are membrane-bound sac-like structures inside the chloroplast. Their lumen holds many electron transport proteins, and their membrane is rich in the light-harvesting complex. Proteins destined for the thylakoids first enter through the general TOC/TIC import pathway. In the stroma, stromal processing peptidases remove the transit signal and expose thylakoid...

protein transport to the stroma

Chloroplast Protein Import into the Stroma

Chloroplast protein import into the stroma depends on a set of membrane systems and protein machines that place each protein in the right location. Chloroplasts have an outer membrane, an inner membrane, and a thylakoid membrane. Each one contains its own transporters, translocons, and enzymes, so sorting is essential for chloroplast function. Two major complexes carry out this import step. The translocon of the outer chloroplast membrane, or TOC complex, works with the translocon of the inner...

protein transport to the outer chloroplast membrane

protein-transport-to-the-outer-chloroplast-membrane

Chloroplast Outer Membrane Protein Import

Chloroplast outer membrane protein import begins in the cytosol, where nucleus-encoded proteins are made. After synthesis, these precursors bind cytosolic factors such as 14-3-3 protein and Hsp70 chaperones. These factors keep the proteins unfolded until they can move to the chloroplast. Two models explain how the precursor is recognized by the TOC complex, which is the outer membrane transport system. In the first model, the new precursor binds TOC159 and forms a complex. That complex docks...

protein transport to the inner chloroplast membrane

protein-transport-to-the-inner-chloroplast-membrane

Inner Chloroplast Membrane Protein Pathways

Proteins reach the inner chloroplast membrane through several import pathways. These proteins, often called plastid proteins, use N-terminal transit sequences and internal targeting sequences to reach the correct chloroplast subcompartment. One route is the stop-transfer pathway. In this pathway, an internal hydrophobic sequence blocks movement into the stroma, the fluid-filled space inside the chloroplast. The precursor pauses across the TIC complex and is then released sideways into the...

key terms and concepts

Explore how cells control protein entry into the nucleus by masking NLS and NES signals, changing receptor binding, adjusting pore size, and processing precursors.

Nuclear Transport Controls Protein Entry | JoVE K12

cell biology

JoVE Core Cell Biology illustrates complex cellular processes in easy-to-understand yet comprehensive animated video lessons. This collection focuses on the essential components and functions of cells, including organelles and membranes, cellular transport, and cell energetics. It is complemented by scientist-in-action videos that demonstrate the application of cell biology in real-world research experiments.

Nuclear Transport Controls Protein Entry

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