Protein Sorting: The Golgi Apparatus Lecture PDF

Protein sorting: the Golgi apparatus BIOL2020 Reading material https://journals.biologists.com/jcs/article/131/5/jcs209890/3138/Form ation-of-COPI-coated-vesicles-at-a-glance Remember the secretory pathway 1. 2. 3. 4. 5. 6. 7. Translates from mRNA in the ribosomes in the cytoplasm Enters the ER...

Protein sorting: the Golgi apparatus BIOL2020 Reading material https://journals.biologists.com/jcs/article/131/5/jcs209890/3138/Form ation-of-COPI-coated-vesicles-at-a-glance Remember the secretory pathway 1. 2. 3. 4. 5. 6. 7. Translates from mRNA in the ribosomes in the cytoplasm Enters the ER lumen Goes from the ER to the Golgi in a vesicle Transits the Golgi Leaves the Golgi in a vesicle The vesicle fuses the cell membrane It is outside Part 1 vesicular transport Transport vesicles bud off from one compartment and fuse with another. As they do so, they carry material as cargo The secretory pathway leads outward from the endoplasmic reticulum (ER) toward the Golgi apparatus and cell surface, while the endocytic pathway leads inward from the plasma membrane. Most transport vesicles form from specialized, coated regions of membranes. They bud off as coated vesicles, which have a distinctive cage of proteins There are four well-characterized types of coated vesicles: clathrin-coated, COPI-coated, COPII-coated, and retromer-coated. Each type is used for different transport steps. Coats are geometrical structures that assemble into vesicle cages cargo membrane Clathrin https://www.youtube.com/shorts/wuaUcAJeb1M Rab Proteins Guide Transport Vesicles to Their Target Membrane all transport vesicles display surface markers that identify them and target membranes display complementary receptors . SNARE proteins Mediate Membrane Fusion by putting the membranes in close proximity Proteins Leave the ER in COPII-coated Transport Vesicles that form at the ER exit site (ERES) Cargo receptors inside vesicles ensure they are loaded ● ● Cargo-specific receptors Activated receptors recruit adaptor proteins that recruit the coat proteins COPII vesicles transport vesicles can accommodate the large cargoes by assembling tubes instead of vesicles Giulia Zanetti 2013 elife Part 2 The Golgi Osmium tetroxide stain The Golgi consists of a collection of flattened, membrane-enclosed compartments called cisternae Glycosylation and phosphorylations at the Golgi are a destination code ● ● ● Sugar coats serve as destination tags Some sugar coats are transient Glycosylation steps are compartmentalized in different cisternae Transport Through the Golgi Apparatus Occurs by Two Mechanisms ● Vesicles transport molecules between cisternae ● Cisternae maturate from cis to trans together with cargo molecules Original evidence - fixed samples Vesicular model ● ● Maturation model Vesicles bud and fuse COPI vesicles surround the Golgi ( 60–90 nm ). ● ● ● Bofanti Cell. 1998 Proto Collagen is bigger than vesicles (300 nm), PC is not detected inside vesicles. It localizes to enlarged Golgi cisternae Recent evidence - fluorescent live imaging These are all fluorescent proteins ● ● ● Make fusion proteins , e.g., Golgi protein fused to GFP Track GFP through the Golgi See processes in real time and in different colors In some species, the Golgi cisternae are not stacked • A system of dispersed cisternae in the case of the budding yeast Saccharomyces cerevisiae Fluorescence microscopy of a single cisternae shows its maturation dynamics cis-Golgi protein(Green) trans-Golgi protein(Red) Papanikou 2015. eLife COPI vesicles bring ER proteins back from the Golgi ER proteins have the KDEL sequence and KDEL receptor initiates vesicle formation Cytoplasm Golgi • KDEL = Lysine - Aspartic acid - Glutamic acid - Leucine A nonselective constitutive secretory pathway transports most other proteins directly to the cell surface. ● ● No specific signal = secrete the protein (default pathway) Specific signals are needed to direct secretory proteins into secretory vesicles and lysosomal proteins into different specialized transport vesicles. The secretory pathway is only a part of the story. There are still the cytoplasmic and nuclear proteins Summary ER The Golgi ● 2 models of anterograde movement ● Retrograde movement Coats ● COPI ● COPII ● Clathrin Functions ● Molecules are sorted at the Golgi ● Sugars are the main destination tag Parts of the Golgi Cisternae CisGN TransGN Lysosome Acidic Hydrolyzing enzymes Membrane Lysosome Vesicles