endomembrane system internal membranes origins transport of proteins chapter 15 endomembrane system

Transcription

1 endo system chapter 15 internal s endo system functions as a coordinated unit divide cytoplasm into distinct compartments controls exocytosis and endocytosis movement of molecules which cannot pass through origins endo system invagination of plasma mitochondria endosymbiosis chloroplasts endosymbiosis transport of proteins protein synthesis begins on a ribosome in cytoplasm sorting signals protein transport nuclear transport protein translocators vesicular transport

2 pathways endocytic endosomes & lysosomes exocytic biosynthetic pathway secretory pathway constitutive secretion regulated secretion secretory granules exocytic secretory granule regulated secretory pathway constitutive secretory pathway endocytic endocytic pathway golgi complex endoplasmic reticulum rough ER cisternae - many bound ribosomes polarity rough ER endoplasmic reticulum smooth ER functions synthesis of steroid hormones liver processing of toxins sequestration of Ca+ sarcoplasmic reticulum biosynthesis in RER synthesis of proteins free ribosomes cytosolic proteins peripheral proteins nuclear proteins bound ribosomes secretory proteins integral proteins Incoming ribosomal subunits Growing polypeptides Start of mrna (5 end) Polyribosome Completed polypeptide End of mrna (3 end) Ribosomes mrna 0.1 μm

3 biosynthesis in RER bound ribosomes bind to ER as needed SRP (signal recognition particle) binds to SRP receptor translocon biosynthesis in RER translocon SRP interacts with a SRP receptor ribosome interacts with the translocon SRP is released facilitated by G-proteins Ribosome 1 SRP ER LUMEN mrna Signal peptide 2 SRP receptor protein Translocation complex 3 4 Signal peptide removed 5 ER Protein 6 CYTOSOL Ribosome 1 SRP ER LUMEN mrna Signal peptide 2 SRP receptor protein Translocation complex 3 4 Signal peptide removed 5 ER Protein 6 CYTOSOL biosynthesis in RER Processing signal sequence cleaved by signal peptidase Carbohydrates added oligosaccharyltransferase RER lumen chaperones assist in folding protein disulfide isomerase adds disulfide bonds to cysteines. biosynthesis in RER integral proteins hydrophobic region generated translocon opens laterally expels trans segment synthesis continues

4 biosynthesis in RER glycosylation begins in the RER bound glycosyltransferases (GTs) lipid carrier (dolichol phosphate) oligosaccharyltransferase -> transfers to polypeptide removal of 2 of 3 terminal glucose residues quality control glycoprotein binds with calnexin (chaperone) terminal glucose removed GTs recognize malfolded proteins tag with glucose --> refolded or destroyed transferred to golgi golgi bodies golgi apparatus functionally distinct cisternae - cis trans differences in composition between cis and trans face trans-golgi network (TGN) trans cisternae medial cisternae cis cisternae cis- Golgi network movement of materials movement across Golgi vesicular transport model vesicles shuttle cargo from CGN to TGN cisternal maturation model cisternae mature as they move through the apparatus combination? retrograde transport via vesicles cisternal vesicular glycosylation carbohydrate assembly glycolipids and glycoproteins assembled across golgi by glycosyltranferases

5 vesicle shuttles vesicle-mediated transport donor compartment acceptor compartment direction anterograde transport toward plasma retrograde transport away from plasma protein coats two main functions cause to curve select components to be carried vesicle shuttles (1) cargo selection sorting mechanisms coat proteins COPI, COPII, clathrin bulk transport proteins trapped Nature Reviews Molecular Cell Biology 10, (May 2009) vesicle shuttles vesicle shuttles (2) budding coat proteins interact with adaptor proteins COPII - sec23/24, sec13/31, Sar1 Sar1 coat recruitment GTPase GTP bound Sar1 is bound sec23/24 COPII-coated vesicle sec13/31 (3) scission scission proteins excise bud (4) uncoating coat proteins detach and get reused protein cargo outer coat inner coat cargo receptors donor

6 vesicle shuttles (5) transport microtubule transport via motor proteins dynactin (6) tethering Rab proteins bind to tether proteins on acceptor Rab vesicle shuttles (7) docking soluble NSF attachment protein receptor v-snares t-snares (8) fusion (9) disassembly NSF and SNAP proteins tethering proteins exocytic pathway TGN proteins sorted secretory (constitutive & regulated) lysosomal constitutive secretion plasma proteins polarized cells - apical / basal proteins segregated in TGN non-lysosomal proteins regulated secretion aggregate as dense granules at TGN secretory vesicles (granules) 2 main functions timing, amount

7 release of materials exocytosis triggered by increase in Ca2 + vesicle fuses with plasma autophagy digestion of organelles autophagosome organelle wrapped in double fuses with lysosome autophagolysosome lysosome enzyme sorting sorting proteins at TGN lysosomal proteins tagged phosphorylated mannose recognized by MPRs released in clathrin-coated vesicles lysosomes digestive organelles contain acid hydrolases digest a wide variety of biological molecules intracellular digestion lysosomal storage disorders absent or defective enzymes buildup of undigested materials Tay-Sachs uptake of materials phagocytosis endocytosis pinocytosis receptor-mediated endocytosis

8 RME receptor-mediated endocytosis clathrin coat arranged in geometric pattern clathrin structure clathrin triskelion three heavy chains three light chains binds to AP2 adaptor binds to cargo receptors lipoproteins on cell surface clathrin polygons on inner surface invagination of clathrin pit clathrin structure clathrin triskelions and adaptors geometrically arranged dynamin-ring promotes pinching off of coated pit into vesicle uses GTP to tighten around neck of vesicle Hsc70 uses ATP and frees clathrin coat endocytic pathway endosomes formed from fusion of endocytes sorts materials arriving from plasma early endosomes derived from PM vesicles fuse with early endosome proteins dissociate - ph affects conformation proton pumps lower ph of endosome late endosome lysosome

9 metabolizing cholesterol cholesterol packaging lipoproteins LDLs and HDLs LDLs taken up by RME lysosomes release cholesterol for use engulfing particulate phagocytosis engulfing large particles phagosome fuses with lysosome phagolysosome fibrous cap endothelial injury macrophage plant cell vacuoles vacuoles aqueous storage organelles tonoplast - water vacuole in plants contains active transport system for K + ions aquaporins