Protein Sorting I Flashcards
What is the plasma membrane? What is its function?
outer boundary of cells, bilayer
function: protective barrier, has transporters, signaling
What is the nucleus? What is its function?
contains the genome
function: principal site for DNA and RNA synthesis
What is the cytoplasm? What is its function?
consists of cytosol and cytoplasmic organelles
function: location of intermediary metabolism
What is the ER? What is its function?
ribosomes attached to it (rough), no ribosomes (smooth)
function: involved in protein synthesis, lipid synthesis, protein folding, storage of calcium
What is the Golgi Apparatus? What is its function?
stacks of disc-like compartments
function: post-translational changes on proteins and lipids, trafficking
What is the mitochondria? What is its function?
outer and inner membrane and matrix
function: makes ATP, signaling, cell differentiation and cell death
What are lysosomes? What do they do?
contain digestive enzymes that degrade organelles and biomolecules
What are peroxisomes? What do they do?
small vesicular compartments that contain enzymes used in oxidation reactions
What are the three topological categories of cells?
nucleus and cytosol; organelles in secretory and endocytic pathways; mitochondria
What is the nuclear envelope?
space between inner nuclear membrane and outer nuclear membrane
What allows the lumen of each topological equivalent organelle lumen to communicate with each other and with the cell exterior?
budding and fusion
This type of transport occurs between nucleus and cytosol through nuclear pore complexes (active transport and free diffusion)
gated transport
This type of transport occurs when protein translocators directly transport specific proteins from cytosol across an organelle membrane
transmembrane transport
This type of transport occurs when membrane-enclosed transport intermediates move proteins between various compartments via vesicles
vesicular transport
Protein transfer/transport to various compartments is guided by what?
sorting signals
In regards to protein sorting signals, what is the typical length of a stretch of amino acids?
15-60 residues
How are protein sorting signals localized?
on N or C terminus or within protein sequence
In regards to protein sorting signals, multiple scattered sequences in protein may form what?
signal patch
What do signal peptidases do?
may remove signal after protein reaches final destination
What are both necessary and sufficient for protein targeting?
signal sequences
True or False?
In terms of proteins sorting signals, physical properties of the sequence are more important than the actual sequence
true
What are signal sequences recognized by?
complementary receptors
This type of transport is gated, bidirectional, and selective
nuclear transport
In terms of nuclear transport, proteins that are needed in the nucleus are imported from where?
the cytoplasm
Where are tRNA and mRNA molecules synthesized and exported to?
synthesized in nucleus; exported to cytosol
What is the molecular mass of a nuclear pore complex (NPC)?
125 million Da.
What are NPCs composed of?
30 different proteins or nucleoporins
How are NPCs arranged?
in octagonal symmetry with one or more aqueous pores
How many NPCs are in the nuclear envelope?
3000-4000
What is the direction of transport molecules in the NPCs?
both directions
What type of diffusion do small molecules utilize in the NPCs?
passive diffusion; facilitated transport
In the NPCs, what is transport facilitated by?
binding of particles to fibrils extending form NPC
What is the size of molecules that travel through the NPCs by active transport?
large
What are Nuclear localization signals (NLS)? What is their function?
short sequences rich in positively charged amino acids lysine and arginine;
function: sorting signals that direct molecules to the nucleus (selective import)
Where are NLS located?
on many different sites on protein; form loops or patches on surface
What recognizes NLS?
nuclear import receptors (NIRs); recognizes a subset of cargo proteins
Where are NIRs found?
cytosol
NIRs bind to NLS on protein and to NPC proteins that are present where?
on fibrils that extend into the cytoplasm
NPC proteins have phenylalanine glycine repays which serve as what?
binding sites for import receptors
How do receptors + cargo traverse NPC?
by binding, dissociating, and re-binding to adjacent phenylalanine repeats
Where do NIR go after cargo is released into nucleus?
return to cytoplasm
In regards to nuclear import receptors, cargo proteins 1, 2, and 3 are involved in direct or indirect binding?
direct
In regards to nuclear import receptors, cargo protein 4 is involved in direct or indirect binding?
indirect
What does nuclear export rely on?
Nuclear export signals (NES) on molecules that need to go out of the nucleus; also need Nuclear export receptors (NER)
How does nuclear export work?
same as nuclear import: NER binds to cargo present in nucleus and NPC proteins; binding, dissociation, re-binding facilitates transport; cargo released in cytoplasm
Where in the cell is the GAP or GTPase activating protein located?
cytosol
Where in the cell is GEF or Guanine exchange factor located?
nuclear
What drives nuclear transport in appropriate direction?
gradient of Ran conformational states
What does Ran-GTP bind to?
complex of import receptor and cargo
What does the binding of Ran-GTP cause?
release of cargo
Where do Ran-GTP and import receptor go after they release cargo?
leave nucleus
What is Ran-GTP hydrolyzed by in the cytoplasm?
Ran-GAP
When the receptor is released from Ran-GDP is it ready for another cycle?
yes
In regards to shuttling proteins, some proteins contain what?
NLS and NES
What is steady state localization of shuttling proteins dependent upon?
relative rate of transport
If rate of import is greater than export, it is considered what?
nuclear
Changing rate of import/export can change what?
location of protein
True or False?
In terms of gene regulatory proteins, transport is loosely controlled
false; stringently controlled
When do gene regulatory proteins enter the nucleus?
only when they’re needed
What is transport of gene regulatory proteins controlled by?
NLS and NES being turned on and off
Gene regulatory protein mechanisms include what?
phosphorylation, proteolysis, binding to inhibitory proteins
How is cholesterol biosynthesis regulated?
feedback regulation
