Intracellular Compartments and Protein Sorting

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...

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 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...

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 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...

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 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...

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...

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...

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,...

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...

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...

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...

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...

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...

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...

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...