Cell Biology Chapter 15 Flashcards
Protein Sorting
The endoplasmic reticulum (ER) and the Golgi complex are sites for protein synthesis, processing, and sorting; Sorting of proteins begins in the ER and early compartments of the Golgi; There are mechanisms to retrieve or retain compartment-specific proteins; The final sorting of material that will leave the Golgi complex occurs in the TGN
Cell Communication through vesicles
Communication between cells is mediated through vesicles; These vesicles pinch off from one compartment, move through the cytosol, and fuse with another compartment in a process called vesicular transport; This constant vesicular traffic also provides the main routes for releasing proteins from the cell by the process of exocytosis and for importing them by the process of endocytosis; Transport vesicles pinch off from the membrane of one compartment and then fuse with the membrane of a second compartment; Transport vesicles deliver soluble cargo proteins, proteins and lipids that are part of the vesicle membrane; they are ferried by transport vesicles from organelle to organelle within the endomembrane system, or to the plasma membrane
General order of transport from synthesis to discharge from the cell
ER; Vesicles; Golgi; Vesicles; Plasma membrane
Endomembrane System Pathways
Secretory – discharge proteins from cells; Constitutive secretion is the continual, ‘unregulated’ discharge of material from the cell; Regulated secretion is the controlled discharge of products, stored in cytoplasmic granules, in response to appropriate stimuli; Endocytic - move materials into cell from extracellular environment; Pinocytosis is non-specific intake of solutions from outside the cell; Receptor-mediated endocytosis refers to uptake of specific extracellular molecules following binding of ligand to receptor
Synthesis of membrane lipids
Most membrane lipids are synthesized by the ER; Enzymes embedded in the cytoplasmic leaflet of ER membrane; Scramblase randomly flips some phospholipids to the opposite leaflet to equally distribute; Various organelles have specific membrane lipid composition achieved by: Golgi selectively flips phospholipids to create asymmetric leaflets (Flippase); Organelles can modify phospholipids; Phospholipids can be selectively transferred to various organelles
Mechanisms for protein transport
Transporting proteins across membranes normally impermeable to hydrophilic macromolecules: 1. Proteins moving from the cytosol into the nucleus use nuclear pores
2. Proteins moving from the cytosol into the ER, mitochondria, or chloroplasts use protein translocators; 3. Proteins moving onward from the ER and from one compartment of the endomembrane system to another use transport vesicles; Transport vesicles pinch off from the membrane of one compartment and then fuse with the membrane of a second compartment; Transport vesicles deliver soluble cargo proteins, proteins and lipids that are part of the vesicle membrane ; Signal sequences are both necessary and sufficient to direct a protein to a particular destination
Proteins enter the endoplasmic reticulum while being synthesized; Proteins destined for the Golgi apparatus, endosomes, lysosomes, and cell surface, ALL first enter the ER from the cytosol; Once inside the ER lumen, or embedded in the ER membrane:They WILL NOT RE-ENTER the cytosol; Instead they are ferried by transport vesicles from organelle to organelle within the endomembrane system, or to the plasma membrane
Free Ribosome synthesis vs Ribosomes target to ER (mechanisms)
All proteins synthesized by ribosomes; All ribosomes located in cytoplasm; ALL PROTEINS ARE initially SYNTHESIZED IN THE CYTOPLASM; Proteins contain sorting signals in amino acid sequence; Direct protein localization; Two separate populations of ribosomes in the cytosol Free ribosomes are unattached to any membrane and are making all of the other proteins encoded by the nuclear DNA; Membrane-bound ribosomes are attached to the cytosolic side of the ER membrane and are making proteins that are being translocated into the ER; Membrane-bound ribosomes and free ribosomes are structurally and functionally identical!; They differ only in the proteins they are making at any given time; When a ribosome happens to be making a protein with an ER signal sequence, the signal sequence directs the ribosome to the ER membrane; Because proteins with an ER signal sequence are translocated as they are being made, no additional energy is required for their transport; The elongation of each polypeptide provides the thrust needed to push the growing chain through the ER membrane
Kinds of Proteins transferred from the cytosol to the ER, how they are synthesized and their fates
Two kinds of proteins are transferred from the cytosol to the ER: 1. Water- soluble proteins are completely translocated across the ER membrane and are released into the ER lumen; The water-soluble proteins are destined either for secretion or for the lumen of an organelle of the endomembrane system; Prospective transmembrane proteins are only partly translocated across the ER membrane and become embedded in it; The transmembrane proteins are destined to reside in the membrane of one of these organelles or in the plasma membrane; All of these proteins are initially directed to the ER by an ER signal sequence; A segment of eight or more hydrophobic amino acids which is also involved in the process of translocation across the membrane; Most of the proteins that enter the ER begin to be threaded across the ER membrane before the polypeptide chain has been completely synthesized; This requires that the ribosome synthesizing the protein be attached to the ER membrane. That’s what gives the appearance of the RER; Soluble proteins made on the ER are released into the ER lumen; Two protein components help guide ER signal sequences to the ER membrane: 1. A signal-recognition particle (SRP), present in the cytosol, binds to both the ribosome and the ER signal sequence when it emerges from the ribosome; 2. An SRP receptor, embedded in the ER membrane, recognizes the SRP; Binding of an SRP to a ribosome that displays an ER signal sequence slows protein synthesis by that ribosome until the SRP engages with an SRP receptor on the ER; Once bound, the SRP is released, the receptor passes the ribosome to a protein translocator in the ER membrane and protein synthesis recommences; The polypeptide is then threaded across the ER membrane through a channel in the translocator; The SRP and SRP receptor unite ribosomes that are synthesizing proteins with an ER signal sequence and available translocation channels in the ER membrane; The signal sequence also functions to open the channel in the protein translocator; This sequence remains bound to the channel, while the rest of the polypeptide chain is threaded through the membrane as a large loop; It is removed by a transmembrane signal peptidase, which has an active site facing the lumenal side of the ER membrane; The cleaved signal sequence is then released from the translocation channel into the lipid bilayer and rapidly degraded; Once the C-terminus of a soluble protein has passed through the translocation channel, the protein will be released into the ER lumen
