Cellular Transport Flashcards
Organelles
Organelle has lumen and membrane
Specific lipid and protein in lumen and membrane (needed for sorting)
Organelle has same function in all cells
Organelle has specific position in cell
Abundance of organelle related to cell type (muscle - energy - mitochondria)
Evolution organelles
Pinching off of plasma membrane, nucleus, Gogli
Endosymbiosis, mitochondria, chloroplasts
Types of membrane associated proteins
Transmembrane (single-, double-, multipass) Membrane associated (one side is hydrophobic) Lipid-linked (lipid anchored) Protein attached (non-integrated (peripheral membrane proteins)
Single-pass membrane proteins
2 signals and one signal
Protein goes to translocation channel, signal peptide is removed, ribosome continues translating until signal sequence is reached (protein is pushed down into translocation channel), channel opens laterally and protein moves into the lipid bilayer
Double-pass protein
2 signals
Targeting, moves towards hydrophobic start-transfer sequence, translocation stops, translation continues, protein moves out of tranlocation channel, translation is finished
Without knowing anything about the protein, you know how it is positioned in the membrane
Hydrophobic in membrane, hydrophilic outside membrane (multipass)
Fate of proteins that have entered the ER
Stay in ER via ER retention signals
Further routing in secretory pathway via vesicular transport
Vesicle traffic
Endocytic pathway
Secretory pathway
Endocytic pathway
Uptake of material from exterior
Degradation in lysosomes
Metabolites to cytosol
2 kinds of vesicle traffic
Phagocytosis, pinocytosis (ordinary and RME)
Phagocytosis
Particles bind to receptors (antibodies) before uptake (triggered process)
Uptake –> large endocytic vesicles (phagosomes) –> fusion with lysosomes –> degradation
Pinocytosis
Uptake via clathrin coated pit –> clathrin coated vesicle –> uncoating –> fusion with early endosome –> late endosome –> lysosome
Ordinary: indiscriminate continuous uptake (retrieval of membrane material)
Receptor-mediated endocytosis): specific binding of macromolecules to receptors –> accumulation in clathrin coated pits –> uptake (LDL)
Sorting in endosoems
Early endosomes –> late endosomes –> develop or fuse lysosomes
Gradual decrease in pH (lowest in lysosome), receptors are released at low pH, recycling, degradation or transcytosis
Cargo always goes to lysosomes for degradation
Secretory pathway
Flow of material towards exterior
ER –> Golgi –> plasma membrane –> external milieu
All transport in secretory pathway via vesicles
During secretory transport
Modification (folding, glycosylation)
Quality control (retention and degradation of misfolded proteins)
Sorting (movement of proteins to other compartments)
Modification (folding, glycosylation)
In lumen of ER, during synthesis, trimming and processing of sugars continues in ER and Golgi –> structure, protection, folding and sorting
Quality control (retention and degradation of misfolded proteins
Only correctly folded proteins can leave the ER
Unfolded proteins remain complexed with chaperones (folding or transfer to cytosol for degradation)
Sorting (movement of proteins to other compartments)
Proteins can stay or move on towards Golgi (default)
Mechanisms to stay:
Aggregation or interaction with chaperones and retention in ER via recognition of retention signal in the protein (return signal)
NB. All ER resident proteins have the retention signal; if they “escape” to the Golgi, they are recognized by a receptor and shuttled back to the ER.
Golgi Apparatus Stacks of cisternae and tubules
Cis golgi network (sorting platform)
Cis cisternae (“powerplant” of glycosylation)
Medial cisternae
Trans cisternae (“powerplant” of glycosylation)
Trans golgi network (sorting platform)
2 pathways of exocytosis
Constitutive secretion (to counteract the uptake of proteins) (present in all cells) Regulated secretion (proteins concentrate, aggregate and packed into vesicles and only fuse if there is a signal (present in specialized cells, storage in vesicles near plasma membrane
Characteristics of vesicles
Transport of both soluble and membrane proteins
Different types of vesicles to ensure correct sorting
Transport must be specific and well-regulated
Specificity of vesicles
Initial recognition by Rab protein that are recognized by tethering proteins, further specificity by v-SNARE in vesicles that docks at t-SNARE in target membrane;many different SNAREs for specific fusion
Undergo conformational modifications, tie vesicle to the membrane so it must fuse (spontaneous fusion)
Steps vescular transport
Donor compartment - budding - fusion - target compartment
Budding: vesicle has a thick protein coat to
shape to faciliate budding (triskelions): clathrin, football like structures that pinch off from donor membrane
Capture molecules to transport (coat assembly and cargo selection, bud formation, vesicle formation, uncoating)
Pinching off by dynamin
Different adaptins dependent on donor membrane
