Bio202 Lecture 6: Vesicular Transport Part 1 PDF

Lecture 6: Vesicular Transport Part 1 https://doi.org/10.1073/pnas.241522198  Today’s agenda - Introduction to vesicular transport and the endomembrane system - The basics of vesicular transport  Almost all proteins are synthesized in the cytosol Figure 1-25...

Lecture 6: Vesicular Transport Part 1 https://doi.org/10.1073/pnas.241522198  Today’s agenda - Introduction to vesicular transport and the endomembrane system - The basics of vesicular transport  Almost all proteins are synthesized in the cytosol Figure 1-25 How are proteins trafficked to the correct cellular compartment? Membrane enclosed organelles import proteins by one of three mechanisms 1) moved fully-folded through a pore 2) moved by protein translocators generally as an unfolded polypeptide chain 3) delivered by transport vesicles Figure 15-4  Vesicle transport - The extracellular space and each of the membrane enclosed compartments that are shaded grey communicate with one another by means of transport vesicles - These membranes are collectively referred to as the endomembrane system ARROWS secretory pathway endocytic pathway retrieval pathway Figure 15-19  Vesicle transport - Vesicles carry soluble proteins, membrane proteins, and lipids between compartments ARROWS secretory pathway endocytic pathway retrieval pathway Figure 15-19  Membrane topology membrane-enclosed organelle e.g., ER or Golgi Lipid bilayer Lumen Outer leaflet Inner leaflet Membrane topology the leaflet INSIDE of the ER or Golgi is the leaflet OUTSIDE of the cell Another way to think about it… the leaflet that faces the cytoplasm in the ER or Golgi faces the cytoplasm in the plasma membrane  Membrane topology The part of protein found in the INSIDE of the ER or Golgi is the part of protein OUTSIDE of the cell N C Another way to think about it… the part of the protein that faces the cytoplasm in the ER or Golgi will also face the cytoplasm in the plasma membrane  Another view of a vesicle fusing with the plasma membrane The membrane leaflet and parts of proteins that face the cytosol in the ER, N C Golgi, and vesicles also face the cytosol in the plasma membrane. cytosol PM extracellular https://doi.org/10.1038/nrm2417  Vesicle transport - Vesicles need to be sorted so they go to the right places - Each organelle must maintain its own identity and protein/lipid compositions ARROWS secretory pathway endocytic pathway retrieval pathway Figure 15-19  The basics of vesicle transport - Transport vesicles bud off a donor compartment and fuse with a target compartment. - Vesicles carry cargo from the donor compartment to the target compartment. - Cargo can be soluble proteins, membrane proteins, and lipids.  The basics of vesicle transport - How do vesicles bud? - How is cargo selected for transport? - How do vesicles recognize the right target compartment? - How do they fuse with the target compartment?  Overview of vesicular trafficking Cell 2004 116, 153-166DOI: (10.1016/S0092-8674(03)01079-1)  The basics of vesicle transport - How do vesicles bud?  How do vesicles bud? morphed EM images of budding vesicles assembly of a protein coat drives the budding process  How do vesicles bud? Figure 15-20 EM images of budding vesicles assembly of a protein coat drives the budding process  Different coats are used for different steps in vesicle transport Show table from the textbook from your textbook COP = coat protein  Different coats are used for different steps in vesicle transport ARROWS secretory pathway endocytic pathway retrieval pathway We will first focus on COPII-mediated ER to Golgi vesicle traffic  Formation of COPII-coated vesicles - The monomeric GTPase Sar1 is responsible for the assembly of COPII coats at the ER membrane - The activity of Sar1 is spatially regulated by an ER-associated Sar1- GEF GEF – guanine exchange factor promotes the exchange of GDP for GTP  Formation of COPII-coated vesicles Sar1-GTP recruits COPII adaptor proteins. COPII adaptor proteins function to… 1) select cargo proteins to be packaged 2) initiate deformation of the ER membrane 3) recruit outer coat proteins which help form a bud Note* - You do not need to memorize all the Sec protein #’s – just know adaptor proteins and outer coat proteins  Formation of COPII-coated vesicles Sar1-GTP recruits COPII adaptor proteins. COPII adaptor proteins function to… 1) select cargo proteins to be packaged 2) initiate deformation of the ER membrane 3) recruit outer coat proteins which help form a vesicle Note* - You do not need to memorize all the Sec protein #’s – just know adaptor proteins and outer coat proteins  The basics of vesicle transport - How is cargo selected for transport?  How does cargo get selected for transport? EXIT SIGNALS - commonly an amino acid sequence on the protein - both soluble and membrane-bound cargo proteins have exit signals - membrane-bound proteins with exit signals are either… 1) destined to reside in the membrane of another organelle in the endomembrane system or 2) act as cargo receptors for soluble proteins and are recycled back to the ER exit signal on a membrane-bound cargo protein  How does cargo get selected for transport? The coat serves two main functions: 1) the inner coat serves to concentrate cargo 2) the outer coat, with its curved structure, deforms the membrane and shapes the vesicle occasionally resident ER proteins get trapped in the vesicle  Overview of vesicular trafficking Cell 2004 116, 153-166DOI: (10.1016/S0092-8674(03)01079-1)  The basics of vesicle transport - How do vesicles recognize the right target compartment? How do vesicles recognize the right target compartment? A sequential targeting system is used to increase the probability of a vesicle fusing with the correct target 1st targeting system: Specific Rab proteins on the surface of each type of vesicle are recognized by corresponding tethering proteins on the cytosolic surface of the target membrane Figure 15-22 How do vesicles recognize the right target compartment? A sequential targeting system is used to increase the probability of a vesicle fusing with the correct target 2nd targeting system: SNAREs on the vesicle (v-SNAREs) interact with complementary SNAREs on the target membrane (t-SNARES) Each transport vesicle-target membrane pair have unique Rab-tether and v-SNARE-t-SNARE pairs Figure 15-22  The basics of vesicle transport - How do they fuse with the target compartment? How do vesicles recognize the right target compartment? In addition to aiding in the docking of a vesicle to the correct target membrane, SNARE proteins catalyze the fusion of the donor vesicle with the target membrane. Figure 15-23 membranes must be brought within 1.5 nm of each other to allow lipids to flow from one bilayer to the other  Once fused with the target membrane, the cargo receptor releases soluble cargo How? Figure 15-22 Next class: - ER exit and quality control - the Golgi - exocytosis