14. MODULE 14.pdf

MODULE 14 Protein Movement Between Compartments proteins are synthesized on free or ER-bound ribosomes in the cytosol or in mitochondria proteins without a sorting signal remain in the cytosol; proteins with sorting signals go elsewhere three mechanisms of transport: (1) gated transport: bet...

MODULE 14 Protein Movement Between Compartments proteins are synthesized on free or ER-bound ribosomes in the cytosol or in mitochondria proteins without a sorting signal remain in the cytosol; proteins with sorting signals go elsewhere three mechanisms of transport: (1) gated transport: between cytosol and nucleus (2) transmembrane transport: membrane-bound translocators from the cytosol into topologically distinct spaces (e.g., into the ER, mitochondria) (3) vesicular transport: vesicles move proteins from one compartment to another Endoplasmic Reticulum (ER) smooth or rough labyrinth of tubules/sacs with a lumen; the rough ER has attached ribosomes, and the smooth ER does not proteins are co-translationally translocated across the ER membrane, embedded in it, or fully translocated in the ER lumen; such proteins are destined for the ER or Golgi (as residents), plasma membrane, lysosomes, or secretion a single mRNA binds many ribosomes (forming a polyribosome); the nascent protein attaches to the rER via a signal sequence the ER lumen and the extracellular environment are separated from the cytosol by a membrane; thus, they are topologically distinct from the cytosol but topologically equivalent to each other Functions of the Smooth ER (sER) without ribosomes synthesis of lipids; in steroid-secreting cells, sER synthesizes steroid hormones (e.g., cortisol, testosterone, and estrogens) oxidation of long-chain and very long (> 22 carbons) chain fatty acids in liver cells, the sER contains enzymes for glycogen metabolism and detoxification in muscle, the sER (“sarcoplasmic reticulum”) stores/releases calcium to stimulate muscle contraction sER: Carbohydrate Metabolism glycogen: multi-branched polymer of glucose; accumulated in response to insulin, broken down to glucose in response to glucagon glycogen is mainly stored in the liver and muscles; it is a source of energy if blood glucose levels decrease in hepatocytes, the sER membranes contain glucose-6-phosphatase that breaks down glucose-6-phosphate to glucose this conversion allows glucose to leave liver cells (through GLUT2) and travel into the blood circulation for uptake by other cells Functions of the Rough ER (rER) synthesis of secretory, lysosomal, integral membrane, and Golgi/ER resident proteins folding of proteins; recognition and removal of misfolded proteins (quality control, ER-associated degradation process) assembly of multimeric proteins post-translational and co-translational modifications such as disulfide bonds, and the addition of carbohydrates to proteins Protein Synthesis on Membrane-bound versus Free Ribosomes proteins synthesized on rER-attached ribosomes: secreted, integral membrane, proteins within the endomembrane system (e.g., ER or Golgi residents, lysosomal proteins) proteins synthesized on free ribosomes: destined for cytosol, peripheral proteins at the inner side of the plasma membrane, peroxisomes, mitochondria, nucleus MODULE 14 Signal Sequences and Patches signal sequence: a continuous stretch of amino acids; could be removed by peptidases after sorting signal patch: a three-dimensional arrangement on the protein's surface sorting signals are recognized by sorting receptors that are reused proteins for synthesis at the rER have an N-terminus sequence of amino acids proteins follow a default pathway. Proteins without sorting (localization) signals, synthesized on free ribosomes remain in the cytosol. Proteins synthesized on rER-bound ribosomes are secreted if they do not have a localization signal other than an N-terminal signal Secretory Proteins a messenger (m)RNA is read by ribosomal complexes the first ~60 aa of the synthesized polypeptide is a “destination” signal the signal is recognized by a signal recognition particle (SRP) that stops the synthesis and relocates the complex to the rER membrane by binding its receptor there the ribosome aligns with a translocator, the SRP-receptor dissociates, and protein synthesis resumes the translocator guides the peptide chain into the lumen, where the signal is removed by peptidase Regulation of Secretory Protein Synthesis the Signal Recognition Particle (SRP) and its receptor are G proteins (i.e., bind and hydrolyze GTP; this is a GTPase activity) SRP and its receptor interact with one another only when both are bound to GTP hydrolysis of GTP bound to these proteins triggers the release of the N-terminal signal sequence by SRP and interaction of the signal into the translocator Integral Membrane Protein Synthesis these proteins do not pass entirely through the ER membrane during translocation have hydrophobic transmembrane segments that are shunted from the translocon into the lipid bilayer the translocon contains a lateral “gate” for the hydrophobic segments of the nascent protein Protein Processing in the rER signal peptidase removes signal peptides (at the N-terminus); chaperones bind unfolded proteins protein disulfide isomerase catalyzes the formation of disulfide bonds (-S-S-); transferase adds N-linked oligosaccharides to proteins the first seven sugars are added to the dolichol-PP on the ER cytosolic side; then the oligosaccharides are flipped to the lumen where more sugars are added the assembled oligosaccharide is transferred to an asparagine residue in the sequence N-X-S/T on polypeptides Putting It Together each organelle has distinct proteins; newly synthesized proteins find their way to the proper organelle by localization (sorting) signals signal sequences and patches are recognized by sorting receptors that deliver proteins to the appropriate organelles proteins without sorting signals are synthesized on free ribosomes and remain in the cytosol the smooth ER synthesizes lipids; carries out carbohydrate metabolism, detoxifies metabolites, alcohol, drugs, and stores/releases calcium ions the rough ER produces lysosomal enzymes, secreted proteins, integral membrane proteins, ER and Golgi resident proteins, and carries out the N-glycosylation of proteins cytosolic, mitochondrial, nuclear, and peroxisomal proteins are synthesized on free ribosomes