Unit 5 Intracellular Compartments and Protein Sorting Flashcards
Nuclear envelope
the nuclear membrane is physically connected to the endoplasim reticulum due to the double membrane nature of nuclear envelope
Three ways to transport proteins in Euk Cells
Synthesis starts on free ribosomes in cytosol
After initation in cytosol, three mechanisms direct transport:
through nuclear pores (how to get into nucleus)
transport (fully or partially) across membrane (how get proteins into mitochondria and ER)
By vesicles (from ER to Golgi)
Signal Sequence
- stretch of amino acids located at N terminus (15 - 60 AAs long)
- dictates which of three pathways protiens move in
- if signal sewuence not present proteins remain in cytosol
Nucleus and Double Membrane
Double membrane is physical extension of outer nuclear envelope membrane
structure of nuclear pores extend ACROSS the double membrane, made up of multiple protein structures that are symmetrical
structures inside pores allow more selectiveness
two sides of pore are different from one another
cytosol side has fibrils, nucleus has basket
Proteins to be imported have nuclear localixation sequence
- distinguishing features
- nuclear localization signal interacts with nuclear import receptor
- nuclear protein have to be associated with nuclear import receptor, then binds, therefore allows passage through nuclear pore and released into nucleus
Nuclear Pores
Allows gatekeepr between movement in both directions
Ribosomes
Transport across Membranes: Mitochondria
- Mitochondrion has its own genome, own ribosomes, RNA polymerase, but only few genes, not enough genetic information to make own proteins,
- Proteins that make functional are made in genome of nucleus,
- protein remain unfolded until after it is translocated through membranes
- Chaperone proteins pull protein in and prevent backsliding
- sequence cleaved off by enzymes
- After translocation signal sequence removed and protein folds and becomes functional
Full Import of Protein into Mitochondria
Translation starts on free ribosomes, finish translation but remain unfolded
Signal sequence is recognized by protein associates by outer membrane
Protein also associated with translocation channel protein (allow passage through in single file)
Lipid environemnt of membrane allow lateral movement of proteins - translocation laterally until adjacent to embedded inner membrane protein
Once inside mitochondrion, signal sequence cealved off by folding peptidases and protein folds
Transport across membranes of Endoplasmic Reticulum
bidirectional sustem
water soluble proteins stranlocated completely across into ER lumen (destined for secretion or lumen of organelle)
transmembrane proteins translocated only partially, (destined for plasma membrane, ER membrane, or membrane of other organelle)
Synthesis of proteins that are destined for Endomembrane System
Most extensive membrane system
mRNA translated to proteins, ribosomes bind forming polyribosomes. Bind to membrane. Coated regions become rough ER
water soluble proteins go through completely, not soluble ones get stuck halfway
signal sequence guids into membrane
SRP complex binds to receptor,
Sequences open channel, Sequence binds, rest of protein channel threads through
Transfer process stopped by another AA stop sequence
Synthesis of all proteins on free ribosomes
Rather than threading from A terminus to C terminus
Sequence involved in binding to ribosomes
Threading in loop like to C terminus
Once threaded through entire protein, signal peptidase comes in an cleaves off signal sequence
How are multi-pass proteins inserted into membranes?
Translocator pass off to side
Moving hydrophobic sequence
Always going to be on cytosolic side
Understanding of simplest case
Starts the same soluble or not
Internal hydrophobic stop sequence
Stitched into membrane
How to move proteins once they are in secretory pathway
Vesicular Transport
allow material to leave and enter cell
move material between endomembrane conpartments
carry soluble proteins (in their lumens) to the plasma membrane for secretion
move membrane proteins (in their membranes) to be expressed on the cell surface
Vesicle traffic
outwards from ER - Golgi - other organelles, plasma membrane
inward, from plasma membrane - edosomes - lysosomes
How are vesicles formed
phospholipid bilayer is self healing structure, physically distort membrane to form bubbles
Clathrin (protein that causes distortion) forms basket that gives vesicle shape
asaptins recognize and capture specific cargo for transport
get to pinch point, dynamin pops off the vesicle, protien coat comes off and now just vesicle
Triskelions
During assemply , they come together and form a closed cage
What happens in the Secretory Pathway
Post translationally modified,
additiona of carbs to side chain of AA, happens in proteins when they are only in ER
Glycosylation in Secretory Pathway
additiona of sugar groups,
functions depends on protein
- protect from degradation
- keep protein in ER until properly folded
- help direct protein to proper organelle (transport signal)
- if displayed on cell surface, cell-cell recognition
Glycosylation in ER
happens because enzymes needed for it are only present in ER and do one-type linked glycosylation
only adds to asparagine AA (N-linked glycosylation)
happens to proteins on lumial side of ER, (transmembrane or not)
How do proteins leave the ER
Protein must be folded correctly before leaving
Chaperone proteins recognize incorrectly folded protein and chaperone them to get to folded state,
quality controlled process allows budding off step
Unfortunatly, chaperone may not be able to properly correct misfolding
Unfolded Protein Response
lumenal sensor detect misfolded states,
trigger cytosolic response because that is where proteins are made
protein cascades that activate synthesis of more chaperone proteins = more proteins in ER, ER physically gets bigger
if this doesn’t help cell dies (apoptosis)
How to proteins get from ER to Golgi
vesicular transport
bud off from ER, fuse with cis side of Golgi
(closest to ER side = cis, closest to plasma membrane = trans)
Vesicle traffic
outward: ER - Golgi - other organelles?plasma membrane?
inward: from plasma membrane - lysosomes
Modification and Sorting in Golgi
series of flattened sacs - cisternae
organized into functionally distinct compartments with cis(entry) closest to ER
