16.1: Translocation of Secretory Proteins Across the ER Membrane

• The rough ER is formed through:

  • Secreted proteins synthesizing

  • Enzymes

  • Golgi complex

  • Lysosome

  • Integral plasma membrane

• They attach to the membrane of the ER and form the rough ER

• ER signal sequences on nascent secretory proteins contain hydrophobic amino acids located at the N-terminus

• The ER binds to the signal recognition particle (SRP) which helps it turn into a nascent secretory protein, which targets the ribosome/nascent chain complex

• SRP and the SRP receptor conduct the insertion of secretory protein in the translocon, and no additional energy is needed for this

• Competed secretory proteins target the ER membrane through interaction with the translocon in post-translational translocation

16.2: Insertion of Proteins into the ER Membrane

• There are four different classes of topology that the synthesized proteins of the integral membrane can fall into on the ER:

  • Topogenic sequences

  • N-terminal signal sequences

  • Internal stop-transfer sequences

  • Internal signal-anchor sequences

• They all direct proteins in the ER membrane

• Membrane proteins that are single-pass contain up to two sequences, while multiple pass ones contain multiple

• Cell-surface proteins can be initially synthesized into type I proteins and cleaved with a luminal domain transfer

16.3: Protein Modifications, Folding, and Quality Control in the ER

• N-linked oligosaccharides contain two N-acetylglucosamine and three mannose

• O-linked oligosaccharides are short and contain one to four sugar residues

• Glycoproteins may be assisted in their folding by oligosaccharide side proteins, along with helping protect mature proteins from proteolysis, work as antigens and help with cell-cell adhesion

• Protein disulfide isomerase (PDI) works as a catalyst for the formation and rearrangement of disfutile bonds in the ER lumen

• The only proteins that are transported into the Golgi complex from the rough ER are properly folded proteins

16.4: Export of Bacterial Proteins

• Completed proteins can be translocated through gram-negative bacteria, and the translocation takes the proteins through a translocon related to eukaryotic cells in the ER

• The SecA protein works as the driver for post-translational translocation through the inner membrane

• Hydrolysis and cytosolic ATP help with bacterial secretion throughout all of the cellular systems

• The pathogenic bacteria use type III secretion apparatus to inject proteins into cells

16.5: Sorting of Proteins to Mitochondria and Chloroplasts

• Nuclear genes take up most of the encoding go of mitochondrial and chloroplast proteins and they also use synthesized ribosomes which are imported into the organelles

• The N terminal uptake targeting sequence contains the information needed to target precursor proteins

• Organelles only take unfolded proteins, which are maintained through cytosolic chaperones

• Proteins transfer to the general import pore once they bind to the receptors on the outer membrane of the mitochondria if they are meant to be in the mitochondrial matrix

• Proteins can have secondary targeting sequences if they are meant to be a part of the thylakoid

• The inner and outer membrane translocation channels help with the import of proteins through the chloroplast stomata channels

16.6: Sorting of Peroxisomal Proteins

• The synthesization of peroximal proteins occurs on cytosolic ribosomes and they are post-translationally incorporated into the organelle

• They contain a C-terminal PTS1 targeting sequence and sometimes an N-terminal PTS2 targeting sequence, which aren’t cleaved after importing

• If a protein is destined to the peroxisomal matrix, then they bind to a cytosolic receptor and they are directed to common import receptors along with translocation machinery along the peroxisomal membrane

• Peroxisomal membrane proteins contain a targeting sequence that is different from the peroxisomal matrix proteins, also, they move along a different pathway