Lecture 13 Flashcards
List the 3 topological compartments, what major organelles/structures they are associated with, and how they communicate
Nucleus and Cytosol: communicate through the “gated transport” of the nuclear pore complex
Organelles in the secretory/endocytic pathways (ER, Golgi, Endosomes, and lysosomes): communicate through “budding and fusion” of vesicles
Mitochondria: not big on communication (keep to themselves bc of the potential danger from their free radicals
In terms of protein trafficking, define Gated Transport
What occurs, via nuclear pore complexes that conduct active and free diffusion, between the nucleus and the cytosol
In terms of protein trafficking, define Transmembrane Transport
When membrane translocators directly transport proteins from the cytosol across a organelle membrane
In terms of protein trafficking, define Vesicular Transport
Membrane-enclosed transport intermediates that move proteins between various compartment via vesicles
Sorting signals are stretches of ________ that are localized on the ______ or ______ within a protein sequence.
AA’s (typically 15-60 residues long) ; N terminus ; C terminus
Define a signal patch in a protein
multiple, scattered sorting signals in proteins
Signal sequences are _____ and _____ for protein targeting
necessary ; sufficient
Describe signal peptidase’s role in protein sorting
signal peptidase may remove the sorting signal once the protein reaches it’s final destination so that it is no longer targeted
What role do complementary receptors play in protein sorting? what is most important about a protein signal sequence?
they recognize signal sequences
the physical properties of the sequence are most important, not necessarily the actual sequence.
(Signal Sequences)
describe the signal sequence that codes for the import of a protein into the nucleus.
Lysine and Arginine rich sequences in ANY order
the sequential positive charges from lysine and arginine is the main physical characteristic that codes for import into the nucleus
(Signal Sequences)
describe the signal sequence that codes for the import of a protein into the Mitochondria.
A combination of positively charged, and hydrophobic AA’s that form a coil
ex. +H3N-Met-Leu-Ser-Leu-Arg-Gln-Ser-Ile-Arg-Phe
(+H3N is the N terminus)
(Arg is positively charged)
(Signal Sequences)
describe the signal sequence that codes for the import of a protein into the ER.
a stretch of hydrophobic AA’s
ex. Leu-Leu-Leu-Val-Gly-Ile-Leu-Phe-Trp
(Signal Sequences)
describe the signal sequence that codes for the return of a protein to the ER.
KDEL is the specific combination that is needed to signal for this ; Lysine A
ex. -Lys-Asp-Glu-Leu-COO-
(COO- is the C terminus)
Describe the characteristics of nuclear transport (3 of them)
gated, bidirectional, and selective
Proteins needed in the nucleus are imported from the _______ where they are synthesized. Alternatively, ______ and ______ molecules synthesized in the nucleus are exported to the cytosol.
Cytosol ; tRNA ; mRNA
Describe Nuclear Pore Complexes (NPC’s) in terms of their composition and structure.
composed of 30 different nucleoporins (a type of protein) arranged in octagonal symmetry or “baskets” with one or more aqueous pores.
List the types of transport that NPC’s conduct, and the significance of the NPC fibrils.
NPC’s conduct bidirectional passive diffusion of small molecules and facilitated transport of larger molecules.
fibrils extend into the cytosolic side of the nuclear membrane and bind to the particles being transported via facilitated transport. This aids in their movement across the nuclear membrane.
Describe Nuclear localization signals (NLS’s)
short sequences of positively charged AA’s (lysine and arginine) that are located on many different sites on a protein and code for the selective import of that protein into the nucleus.
What recognizes nuclear import signals (NIS’s)? describe this.
Nuclear import receptors (NIR’s), which are soluble cytosolic proteins that bind to NLS’s (Nuclear localization signals) and to NPC (nuclear pore complex) proteins present on their fibrils.
NPC proteins have ________ which serve as binding sites for import receptors.
phenylalanine glycine (FG) repeats
describe how receptors and their protein cargo traverse NPC’s.
receptors plus their cargo move through the NPC by binding, dissociating, and re-binding to adjacent FG repeats until the cargo can be released inside of the nucleus and the NIR (nuclear import receptors) then returns to the cytoplasm.
(sort of “crawls” by binding over and over to the NPC FG repeats)
Describe what Ran-GAP and Ran-GEF are, and where they are found in the cell.
Ran-GAP exists in the cytosol and removes a phosphate from Ran-GTP to create Ran-GDP
Ran-GEF exists in the nucleus and adds a phosphate to Ran-GDP to create Ran-GTP
(both are Ran specific regulatory proteins)
Describe the mechanism of nuclear import of a protein cargo and what happens to the nuclear import receptor.
the cargo protein bound to a nuclear import receptor enter the nucleus, and the protein cargo is released when Ran-GTP binds to the nuclear import receptor.
The Ran-GTP complex then moves out of the nucleus and is converted, by Ran-GAP, to Ran-GDP which then releases from the nuclear import protein.
Describe the mechanism of Nuclear export of a cargo protein and what happens to the nuclear export receptor.
The nuclear receptor enters the nucleus through the NPC, once in the nucleus, Ran-GTP and the cargo protein bind to the nuclear export receptor.
Ran-GTP is converted to Ran-GDP and a phosphate by Ran-GAP hydrolyzing it.
the Ran-GDP and protein cargo are released in the cytoplasm
