Intracellular Membrane Traffic Flashcards
Proteins shuttled from cytosol to nucleus and vice versa
Via gated transport
Protein Translocation
When proteins move from different membrane bound organelles. EX. Plastids, Mitochondria, Peroxisomes, ER
Vesicular Transport
Proteins transported by vesicles ER to Golgi and vice versa. Late endosome, Lysosome, Early Endosome, Secretory Vesicles. Going to cell exterior
Transport Vesicles
Bud off from one compartment (donor) and fuse with another (target). Carry material as cargo from donor compartment to membrane of the target compartment.
Vesicle Budding
Requires a membrane fusion event triggered from the lumenal side of the membrane.
Vesicle Fusion
Requires a fusion event initiated from the donor and target membranes.
Different coats used in Vesicular Transport
Select different cargoes and have different transport paths that serve diff functions.
Clathrin, COPI, and COPII
Structure of Clathrin Coat
Triskelion Structure made from 3 heavy and 3 light chains. Multiples of 3 pentagons, hexagons, sometimes heptagons resemble deformed soccer balls
Assembly/ Disassembly of Clathrin Coat
The assembly of the coat introduces curves in the membrane, leading to uniformly sized coated buds. Adaptor Proteins bound to specific cargo receptors bind to clathrin triskelion’s.
After the clathrin vesicle is formed its coat is lost rapidly. Uncoating regulated by cytoplasmic protiens and pinching off.
Return from plasma membrane endosomes, Golgi
Rab Proteins Guide Vesicle Targeting
Rab(GTPases) direct vesicle to specific spots on the correct membrane by interacting with Rab effectors on surface. Establish some connection not really strong.
Ex. Rab proteins can be large multiprotein complexes or long fibrous tethering proteins.
Specific organelles have specific Rab effectors on thier surface that respond to specific Rab proteins.
SNARE protiens
Help to dock the Rab directed vesicle to the target membrane and catalyze the fusion of the membranes
Ran GTPase also
Imposes directionality on transport through NPC (Nuclear Pore Complexes)
Small GTPase Ran functions like a witch present in ON/ active form (Ran-GTP) or OFF/inactive (Ran- GDP)
Ran GAP activating protein in cytosol (Takes phosphate off ) GDP
Ran GEF located in nucleus binds to chromatin (Puts on to make GTP)
Recruitment of Cargo molecules into ER transport vesicles
By binding indirectly or directly wit the COPII coat membrane and soluble cargo proteins become concentrated in the transport vesicle. Some proteins that COPII binds can help act as cargo receptors bring more proteins to package in the vesicle.
Many diff types of proteins.
Formation of COPII Vesicle
Membrane bound active Sar-GTP recruits COPII subunits to the membrane.
COPI
Mediates the budding of vesicles that return to the ER from Golgi and vesicular tubular clusters.
Usually ER resident proteins. Via KDEL receptor in vesicular tubular clusters recruit budding COPI
Compartmentalization of the Golgi
Oligosaccharide processing happens in Golgi compartments. Processing enzymes not restricted to a particular cistern, graded across the stack. Early acting enzymes first in cis, later acting in trans cistern.
Golgi Struc.
cis network
Golgi Stack - Cis cistern, medial cistern, trans cistern
trans network
Oligosaccharide Processing Pathway in Golgi
Highly ordered, so that each step shown depends on the previous one
Transport through the Golgi
Individual and small proteins transported through vesicle- mediated transport or cisternal maturation
Large proteins through cisternal maturation
Three pathways of Protein Sorting in the trans Golgi network
1) Proteins with the mannose 6- phophate (M6P) marker are diverted to lysosomes(via endosomes) in clathrin coated vesicles.
2) Proteins with signals directing them to secratory vesicles are concentrated in vesicles as a part of a regulatory secretory pathway in specialized secretory cells.
3) In unpolarized cells, a constitutive secretory pathway delivers proteins with no special features to the cell surface
Transport from Trans Golgi
Cell Exterior Exocytosis
Receptor Mediated Endo cytosis of LDL
LDL disassociated from receptor in acidic environment of early endosome LDL in lysosome degraded to give free cholesterol.
LDL receptors returned to plasma membrane clathrin coated vesicle transport.
Endosome Pathway
Maturation of early to late endosomes occurs through the formation of multivesicular bodies, contain large amounts of invaginated membrane and internal vesicles.
Multivesicular bodies are transported inward by microtubules by anchor proteins. As they progress downward they increase maturation to late endosomes, when late no longer send vesicles to the plasma membrane.
Lysosomes are delivered materials for degradation
Through multiple pathways phagosome, autophagosome, endosome
Mannose 6- phosphate receptor
Sorts lysosomal hydrolases in the trans golgi network adds mannose 6-phosphate (M6P) groups to precursors of lysosomal enzymes. Segregate from other types of proteins in the TGN go into clarthin coated vesicle.
The vesicle goes to the endosome low pH receptors disassociate from the receptor, empty receptors are recycled in retromer-coated vesicles to Golgi apparatus. Phosphate removed from the mannose sugars attached to the hydrolases, ensuring that the hydrolases do not return to the Golgi apparatus with the receptor
