anat5.txt

• 1.  COPII - anterograde transport from the ER
• 2.  COPI - retrograde transport from the Golgi and the ERGIC
• 3.  Clathrin - transport from plasma membrane and endosomes

• large protein cages that are spherical in shape
• 1) recruit cargo
• 2) drive vesicle formation - requires energy

• 1.  Initiation and Cargo Selection - recruitment and activation of GTPase to membrane recruits coat proteins and associated cargo to budding site; GTPase is activated when it binds GTP
• 2.  Budding - coat assembles into spherical cage driving vesicle formation
• 3.  Scission - vesicle is released
• 4.  Uncoating - hydrolysis of GTP releases coat - recycled for additional rounds of budding
• 5.  Tethering - "long range interactions" tethering proteins (Rab GTPases and other tethering factors) target vesicles to correct target membrane
• 6.  Docking - "short range interactions" SNAREs on both vesicle and target membranes engage and drive fusion
• 7.  Fusion - the actual fusing of lipid bilayers between vesicle and target

• A SNARE is an alpha helical membrane protein that interacts to drive membrane fusion.
• v-SNAREs are on the vesicle membrane while t-SNAREs are on the target membrane

One v-SNARE interacts with with three t-SNAREs to form a trans-SNARE complex (two sets of complexes per vesicle).  They interact to form a coiled-coil structure which brings the vesicle very close to target membrane.  After fusion, the trans-SNARE complexes are now cis-SNARE complexes because all the proteins are on the same membrane.  The cis-SNARE complexes must be unwound to recycle the proteins- requires GTP.

• 1) Endocytosis- process of removing membranous material from the PM via vesicle budding
• 2) Exocytosis - process of adding membranous material to the PM through vesicle fusion

• 1) Pinocytosis - fluid, uses small vesicles, accounts for most of the endocytosis in most cells
• 2) Phagocytosis - large particles ie. macrophages ingesting bacteria
• 3) Receptor-Mediated - cell signaling, clathrin coat-mediated

• 1) Lysosomal enzyme is phosphorylated on mannose in cis Golgi
• 2) M6P receptor binds enzyme and recruits clathrin
• 3) Enzyme receptor complex buds from Golgi
• 4) Vesicle uncoats
• 5) Vesicle fuses with the late endosome (pH 5)
• 6) Acidic pH of late endosome releases enzyme from M6P receptor
• 7) M6P receptor recycles back to trans Golgi
• 8) Enzyme is dephosphorylated and traffics to lysosome

• 1) Secreted lysosomal enzyme binds M6P receptor on PM
• 2) Enzyme receptor complex recruits clathrin and buds into cell
• 3) Vesicle uncoats and fuses with early endosome (pH 6)
• 4) Early endosome fuses with the late endosome (pH 5.5)
• 5) Acidic pH of late endosome releases enzyme from M6P receptor
• 6) M6P receptor recycles back to trans-Golgi or (7) PM
• 8) Enzyme is dephosphorylated and traffics to lysosome 

• Vital to breaking down/recycling cellular components
• Acidic pH=5
• Receive hydrolytic enzymes from Golgi
• Receive material to degrade from endocytic pathway, phagocytosis, and autophagy

• material that can't be completely digested by lysosomes, occurs in long living cells
• called lipofuscin in neurons
• cells can exocytose the whole lysosome to get rid of the residual bodies

• Tay-Sachs Disease- hexosaminidase A (Hex-A) enzyme deficient
• lipid ganglioside GM2 accumulates abnormally in cells, especially in in the nerve cells of the brain

• Xanthomas- deposits of cholesterol in skin in patients with LDL receptor deficiency
• LDL receptor is not trafficked to PM or endocytosed properly; cells can't uptake LDL and therefore can't catabolize cholesterol

• 1) Vesicular transport
• 2)hidden within lipid transfer proteins (LTP's) = protein carriers for lipids in the cytoplasm (most often across short cytoplasmic gaps between membranes that are in close contact (ie. at ER-Junctions)

• 1) Glycerolipids - has a glycerol backbone, and a hydrophilic choline
• 2) Sphingolipids - has a sphingosine instead of a glycerol backbone; some still have a choline attached like glycerolipids, while others have a sugar attached
• 3) Sterols eg. cholesterol

• 1. LTP binds donor membrane to load cholesterol into the LTP
• 2. LTP dissociates from the donor membrane and diffuses to acceptor membrane
• 3. LTP binds the acceptor membrane, into which it unloads the cholesterol. Targetting to membranes depends on other lipids and proteins, enabling high specificity of transport
• 4. LTP dissociates from the acceptor membrane so that it can used to transport another lipid
• This is an ATP-independent process because lipids are transported down a concentration gradient.

• soluble proteins - no transmembrane domains
• hydrophobic on the inside and hydrophilic on the outside
• bind lipids and proteins in target membranes
• extract lipids from membranes by burying them in a deep hydrophobic pocket 
• multiple large families of LTPs - allows for trafficking of different lipids and to different intracellular locations

• ER Junction - regions in the cell where the ER comes very close to another organelle membrane
• stabilized by bridging complexes
• distance between membranes is 10-50 nm
• membranes do not fuse
• have been identified at the plasma membrane, mitchondria, Golgi, and chloroplast
• LTP binds to both the donor and acceptor membranes simultaneously, and lipids are transported by the LTP swinging back and forth between membranes (no diffusion)