Protein targeting
Supporting his hypothesis, Blobel discovered that many proteins have a short amino acid sequence at one end that functions like a postal code specifying an intracellular or extracellular destination.
[10] In addition to intrinsic signaling sequences, protein modifications like glycosylation can also induce targeting to specific intracellular or extracellular regions.
Even though most secretory proteins are co-translationally translocated, some are translated in the cytosol and later transported to the ER/plasma membrane by a post-translational system.
[20] In addition, proteins targeted to other cellular destinations, such as mitochondria, chloroplasts, or peroxisomes, use specialized post-translational pathways.
[22][23][24][25] Unfolded proteins bound by cytosolic chaperone hsp70 that are targeted to the mitochondria may be localized to four different areas depending on their sequences.
Proteins destined for the mitochondrial matrix have specific signal sequences at their beginning (N-terminus) that consist of a string of 20 to 50 amino acids.
The sequences have a unique structure with clusters of water-loving (hydrophilic) and water-avoiding (hydrophobic) amino acids, giving them a dual nature known as amphipathic.
If mutations occur that mess with this dual nature, the protein often fails to reach its intended destination, although not all changes to the sequence have this effect.
[2][23][29] The general import core (TOM40) then feeds the polypeptide chain through the intermembrane space and into another translocase complex TIM17/23/44 located on the inner mitochondrial membrane.
[25][26] The push and pull of the polypeptide from the cytosol to the intermembrane space and then the matrix is achieved by an electrochemical gradient that is established by the mitochondrion during oxidative phosphorylation.
[25][31] It is this negative potential inside the matrix that directs the positively charged regions of the targeting sequence into its desired location.
If instead the precursor protein is designated to the intermembrane space of the mitochondrion, there are two pathways this may occur depending on the sequences being recognized.
However, once bound to the inner membrane the C-terminus of the anchored protein is cleaved via a peptidase that liberates the preprotein into the intermembrane space so it can fold into its active state.
[2][25] One of the greatest examples for a protein that follows this pathway is cytochrome b2, that upon being cleaved will interact with a heme cofactor and become active.
[24][25] For preproteins containing hydrophobic internal sequences that correlate to beta-barrel forming proteins, they will be imported from the aforementioned outer membrane complex TOM20/22 to the intermembrane space.
General import for the majority of preproteins requires translocation from the cytosol through the Toc and Tic complexes located within the chloroplast envelope.
This delivery process to the stroma is currently known to be driven by ATP hydrolysis via stromal HSP chaperones, instead of the transmembrane electrochemical gradient that is established in mitochondria to drive protein import.
The last of the three are post-translational pathways originating from nuclear genes and therefor constitute the majority of proteins targeted to the thylakoid membrane.
Similarly, nuclear export receptors help move proteins and RNA out of the nucleus using a different signal and also harnessing Ran's energy conversion.
[16] The endoplasmic reticulum (ER) plays a key role in protein synthesis and distribution in eukaryotic cells.
It's a vast network of membranes where proteins are processed and sorted to various destinations, including the ER itself, the cell surface, and other organelles like the Golgi apparatus, endosomes, and lysosomes.
Firstly, a signal-recognition particle (SRP) in the cytosol attaches to the emerging protein's ER signal sequence and the ribosome itself.
[16][20] The initial stages are similar to soluble proteins: a signal sequence starts the insertion into the ER membrane.
However, this process is interrupted by a stop-transfer sequence—a string of hydrophobic amino acids—which causes the translocator to halt and release the protein laterally into the membrane.
[16][20] This results in a single-pass transmembrane protein with one end inside the ER lumen and the other in the cytosol, and this orientation is permanent.
It uses the hydrogen peroxide generated in the earlier reaction to oxidize various other substances, including phenols, formic acid, formaldehyde, and alcohol.
[37] Additionally, catalase within peroxisomes can break down excess hydrogen peroxide into water and oxygen and thus preventing potential damage from the build-up of H2O2.
[42] The cycle for pex5 mediated import into the peroxisomal matrix is restored after the ATP dependent removal of ubiquitin and is free to bind with another protein containing a PTS1 sequence.
Besides the plasma membrane the majority of prokaryotes lack membrane-bound organelles as found in eukaryotes, but they may assemble proteins onto various types of inclusions such as gas vesicles and storage granules.
In most gram-positive bacteria, certain proteins are targeted for export across the plasma membrane and subsequent covalent attachment to the bacterial cell wall.
