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Chapter 16 - Moving Proteins into Membranes and Organelles

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

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