Spatial organization Flashcards
importance of membrane trafficking
- communicate with other cells
- acquire resources
these require control and dynamic changes to the plasma membrane
roles of the
a. endoplasmic reticulum
b. plasma membrane
c. lysosome
a. makes proteins
b. acts as a barrier
c. breaks down proteins
basic principles of the biosynthetic-secretory and endocytic pathways
- polarized trafficking routes all throughout the system
- sorting stations
- retrieval mechanisms and general balance among routes
constitutive secretory pathway
functions in all eukaryotic cells (see lec 1 for diagram)
trans golgi network –> newly synthesized proteins and membrane lipids –> unregulated membrane fusion –> extracellular space
regulated secretory pathway
signal-induced pathway for specialized eukaryotic cells
trans golgi network –> secretory vessel sorting secretory proteins –> signal (i.e. hormone/neurotransmitter) –> intracellular signaling pathway –> regulated membrane fusion
- vesicles stored until a signal triggers their docking and fusion
mature secretory vesicle
secretory vessel made from retrieving golgi components and concentrating cargo
extra plasma membrane
regulated secretion gives extra plasma membrane when needed
- cleavage furrow- one cell dividing into two
- phagocytosis- cell membrane forms a vesicle around organism (tends to be endosomes)
- wound repair (tends to be lysosomal fusing)
basic steps of endocytosis
- invagination- forming a cavity or pouch as the membrane indents into the cytosol
- fission
- endocytosed vesicle joins the early endosome compartment and is routed to other destinations
a. recycling- basolateral domain of plasma membrane
b. transcytosis- to apical domain of plasma membrane
c. degradation (to lysosome)
explain how cells collect resources through endocytosis
- endocytosis
- uncoating of clathrin
- fusion with endosome
A. - budding off of transport vesicles
- return of receptors to plasma membrane
B.
- low pH causes separation and degradation
- trans-cytosis if moved to other side of the cell
fusion
when vesicles merge or fuse with ANY membrane
- SNARE proteins specify which membranes fuse and conduct the process
invagination
making ANY vesicles you invaginate go into the cytosol
- makes an indent in the membrane
- driven by clathrin
coat assembly and cargo selection –> bud formation –> vesicle formation –> uncoating
- also driven by COPI and COPII
budding
indent out of the cell and taking some membrane with it
- how some viruses leave the cell
- driven by the ESCRT complex
ESXRT-0 –> ESCRT-1 –> ESCRT-II –> ESCRT-III (builds up around proteins, causing binding to occur) –> ESCRT-III
SNARE proteins
v-SNARE on the vesicle bind
t-SNARE on the target membrane
- bind to specific SNAREs
clathrin triskelion
clathrin molecule made of 3 heavy chains and 3 light chains
dynamin and fission
dynamin drives fission after vesicle invagination events
- dynamin is necessary to finish clathrin-coated vesicles
cargo is regulated by
signal sequences/moieties
transport machinery is regulated by
- signaling lipids (PIPs)
- small GTPases
- other mechanisms
phospholipid changes
- inositol sugar head can be phosphorylated
- phosphotases and kinases add/remove phosphate groups at different positions of the ring to make a variety of phosphoinsositide (PIP) species
- each PIP binds to specific proteins
- protein partners are recruited to the sites PIPs are found at in the trafficking network
activation of GTPases
- active in the GTP-bound state
- localized in the cell
- bind and activate downstream effectors
rab11 location
recycling endosomes
rab5A location
plasma membrane, clathrin-coated vesicles, early endosomes
rab7
late endosomes
rab functions
- recruited to specific membranes by RabGEFs
- activation promotes downstream effects
- specific Rab works with specific PIP
2 main tissue categories
- epithelial tissue
- lines surface cavities
- surrounds organs
- cells directly connected
- mechanical stresses are transmitted from cell to cell by cytoskeletal filaments anchored to cell-matrix and cell-cell adhesion sites - connective tissue
- cells dispersed
- a lot of ECM provides overall structure
- ECM directly bears mechanical stresses of tension and compression
both are separated by the basal lamina (specialized ECM)
occluding junction
APICAL
tight junction seals gap between epithelial cells
cell-cell anchoring junctions
APICAL
- adherins junction connects actin filament bundle in one cell with that in the next (DO NOT go through the junctions)
- desmosome connects intermediate filaments in one cell to those in the next cell (DO NOT go through the junctions)
channel-forming junctions
BASAL
gap junctions that allow the passage of small, water-soluble molecules from cell to cell
cell-matrix anchoring junctions
BASAL
- actin-linked cell-matrix adhesion anchors actin filaments in ell to extracellular matrix
- hemidesmosome anchors intermediate filaments in a cell to ECM
adheren junction structure
- form strong continuous adhesion belts
- adhesion mediated by cadherin clusters
cadherin mediate through:
- homophilic interactions
- 38.5nm
- C-terminal on outside; N on inside
- Ca2+ keeps rigid for proper adhesion - links to the actin cytoskeleton
- linked by a chain of catenin and other anchor proteins
adheren function
tissue maintenance during development
- tissue structure is lost in adherens junction mutants
tumour suppression
- loss of epithelial structure is a hallmark of cancer
apical and basal sides
- apical faces the lumen or animal surface
- basal faces underlying tissue
- creates polarity for organ function
importance of polarity
controls the solute diffusion between our body compartments
- molecules are blocked from diffusing between cells by tight junctions
- must be actively transported through cells by plasma membrane channels allowing for precise regulation
what would happen without tight junctions?
- molecules can travel back and forth
- membrane proteins could diffuse
- passive carriers diffuse to apical side
- loss of polarity
how are tight junctions formed
- formed by the interaction of transmembrane proteins
- these pass through the cell 4 times
core tight junction proteins
- claudin
2. occludin
what movements do tight junctions block?
- movement of aqueous molecules through the extracellular space b/w cells
- movement of membrane molecules between the apical and basolateral domains of each cell’s plasma membrane
how is polarity established
cells use landmarks (type of signal or structure) to establish and elaborate polarity
what do chemoattractants do?
polarize cells
- bind to a receptor causing both actin polymerization and actin-myosin contraction
microtubules
- 13 parallel protofilaments forming a hollow cylinder
- inherently polarized
protofilaments
- made of heterodimers of alpha and beta-tubulin
- each heterodimer is asymmetric
- both bind to GTP
- they assemble head to tail to form polarized filaments
how do gamma-tubulin complexes nucleate microtubules
- binds tubulin heterodimers to assemble protofilaments into tubes
- gamma-tubulin nucleates microtubules at their minus ends
- plus ends grwo away from nucleation sites
- gamma-tubulin often associates with large microtubule organizing centres
the centrosome
contains 2 centrioles surrounded by hundreds of proteins with gamma-tubulin nucleation sites on the surface
dynamic instability (definition)
single microtubules switch between growing and shrinking
- allows microtubules to search the cytoplasm
dynamic instability (process)
- growing microtubules have a protective cap of GTP-bound tubulin
- plus sides bind to each other
- rapid growth with GTP-capped end
- loss of GTP cap
- rapid shrinkage
- regain of GTP cap
- rapid growth with GTP-capped end
how is motor activity polarized?
- dynein moves to microtubule, minus ends to nucleus
- inesin moves to microtubule, plus ends away
what is a likely key for golgi positioning?
dynein
actin skeleton- cell structure and behaviour
- actin skeleton is inherently polarized from subunit, to filaments, to networks
- actin monomers are asymmetric
- actin monomers bind and hydrolyze ATP
- actin monomers assemble head to tail forming polarized filaments
ATP-ADP polarity in actin cytoskeleton
- after polymerization, Actin-ATP –> Actin-ADP
- hydrolysis reduces binding affinities to neighbouring subunits increasing dissociation
- rate of addition of Actin-ATP > rate of removal of Actin-ADP –> a cap of Actin-ATP can be formed
treadmilling
maintaining a constant length with a flux of subunits through the filament
- subunits can undergo net assembly at + end (=/> net disassembly at - end) because of the polarity
- requires traction to drive cells forward
how is the i end stabilized
the actin-related protein (ARP) complex nucleates actin filaments
- actin filaments are branched to form polarized 2-D networks
- positive at the top (ATP addition)
- negative at the bottom (ADP loss)
how does treadmilling produce protrusive power
- whole networks and single filaments can both treadmill
- net filament assembly at leading edge and disassembly behind leading edge
- caused by diffusion of actin monomers
creating protrusive machines from treadmilling microfilaments
- stationary anchor binds one part of the filament
- treadmilling filament extends from that point
- extension pushes against the cell membrane driving cell protrusion
- protrusive machines are created by anchoring large regions of actin networks
- active filaments hook to transmembrane protein to push against ECM
integrins connect the ____________ to ___________
integrins connect the actin cytokeleton to extracellular matrix molecules
- main receptors that bind extracellular molecules
- transmembrane heterodimers of non-covalently associated alpha and beta subunits
- linked to the actin cytoskeleton via adapter proteins
