Lecture 5 Flashcards
Protein Retention and Retrieval
Specificity of Protein Transport
Inclusion: only proteins interacting with coat is packaged
Exclusion: proteins with signals are not packed
Bulk Flow Model
-Secretion is the default pathway and signals would keep it from being swept away
-example that proves this wrong is beta-lactamase which has a signal for secretion
How was Bulk Flow tested?
Tripeptide Experiment: tracked N-linked oligosaccharide labeled with radioactive tripeptide, they then measure by recovering tripeptide
this was unfair since they did not take into account the folding of proteins
Pulse Labeling
pulse and chase models
-biosynthetic labeling
-labeling of surface or endocytic compartments
-detecting changes use Immunoprecipitation for tracking
Immunoprecipitation
How are they quantified?
isolating proteins using antigen antibody interaction
-subcellular fractionation - movement between compartments
-molecular weight shift - glycosidase
-fluorescence
KDEL
- c-terminal signal for ER localization
- necessary ( if deleted, protein gets secreted)
-sufficient( if added to a secreted protein, it stays in ER)
Retrieval vs Retention
Retrieval is like an open prison, they can go anywhere but is still brought back
Retention is like a closed prison, they are anchored by a signal.
Retrieval Model
Hypo: There’s a receptor that binds to KDEL proteins and retrieve them back
Evidence: There are KDEL proteins that seem to have modified Glc-NAc indicating that they’ve been to Golgi and encountered an enzyme and was retrieved back to ER (Cathepsin D)
If BiP is not anchored in ER but is still free to move in ER, does it disprove the retention hypothesis?
BiP is remains in ER because COPII vesicles excludes them as they have low affinity for ligand-bound KDEL receptors at pH 7.2
KDEL Receptors
KDEL receptors bind in a pH-dependent way
How KDEL receptors bring back runaway ER proteins back to ER?
Receptors that are not in ER are protonated with acidic pH and readily binds with KDEL sequences and COPI vesicles. When ER proteins are in the COPI, they go back to ER. ER proteins are released to ER as receptor becomes deprotonated with ER ph 7 which now readily binds to COPII vesicles.
Other ER retention signal sequences
KKxx and KxKxx are arginine-based sequences that are important for transmembrane proteins. These motifs are recognized by the coatemer of COPI.
Type I proteins have the motif on the C-terminal on the cytosolic side.
Type II proteins have the motif on the N-terminal on the cytosolic side with K/HDEL on the C-terminal sequence on the ER lumen.
Evidence supporting receptor-mediated transport
Secretory proteins are concentrated during ER export in COPII vesicles compared to ER.