Week 10 (Cellular Compartments I Sorting) Flashcards
How do proteins know where to go?
➢Destination depends on amino acid sequence
•sorting signal =address
➢Most proteins lack sorting signal
•remain in cytosol
➢Others have more than one signal sequence
• mitochondria, peroxisomes
•ER (→ golgi → other destinations)
Topological relationship: what is the ECS topologically equivalent to?
Extracellular space is topologically equivalent to: lumen of ER lumen of Golgi lumen of vesicles perinuclear space
Cytosol = inside nucleus
What is the function of Signal Sequences
➢Amino acid sequence •adds address to protein ➢Recognised by receptor proteins (complementary) ➢Continuous or discontinuous ➢Can be removed after sorting by enzymes •signal may not be part of final protein
What are the 3 mechanisms to move protein from one compartment to another?
- Gated transport
- Transmembrane transport
- Vesicular transport
Explain Gated Transport
➢Movement between topologically equivalent spaces
•No need to cross membranes
➢Cytosol ↔ nucleus
➢Guided by signal sequences
➢Signal sequence recognised by receptor protein guides protein across nuclear pore complex
➢bidirectional movement
•e.g. ribosomal subunits
Gated Transport: explain the action of nuclear import/export proteins
➢Nuclear pore complexes (NPCs)
•over 30 proteins “nucleoporins”
➢Nuclear Import Receptors
•soluble cytosolic proteins
•recognise nuclear localisation sequences (signal sequence directing protein to nucleus)
•deliver proteins to nuclear pore for transport
➢To transport out of nucleus = reverse
•nuclear export signal sequence
•nuclear export receptors
Transport across nuclear membrane: size cut off
- Small molecules (<60kDa) can diffuse through the gates
- Large molecules need active transport
- DNA/RNA polymerases (100-200kDa)
- Ribosomal subunits (30nm)
Transport across nuclear membrane: nuclear import receptors
➢Soluble Nuclear import receptors (importins) recognise and bind to nuclear localisation signal (NLS) and they bind nuclear pore complex proteins and they guide movement of proteins through the gates
➢Soluble Nuclear export receptors (exportins)
➢Nuclear export works like nuclear import, but in reverse
Describe Transmembrane Transport
➢Transports across membranes ➢Membrane-bound translocators ➢Unfolded proteins snake through translocators ➢Cytosol → mitochondria ➢Cytosol → ER
Endoplasmic Reticulum
➢Rough ER: coated with ribosomes
➢Membrane separates lumen from cytosol
➢Captures proteins from the cytosol during synthesis
ER: Soluble proteins
- transported fully across the membrane
* destined for secretion or to be inside other organelle
ER: Transmembrane proteins,
- partly transferred across the membrane
- remain embedded in membrane
- destined to reside in a cellular membrane
How are proteins transported into ER
➢Co-translational for ER and postranlational for other organelles
➢N-terminal signal sequence (made first)
➢Signal sequence recognized by (soluble) signal recognition particle (SRP)
➢SRP binds to SRP receptor on ER membrane
➢Polyribosomes attracted to ER (RER)
➢Complex (including the ribosome) brought to translocator
➢SRP-SRP receptor release
➢Translocator transfers growing polypeptide chain through the membrane
Are Soluble Proteins fully transported across the membrane?
➢Soluble proteins are fully transported across the membrane
➢Signal peptidase cleaves off signal sequence
Transmembrane Protein
➢Transmembrane proteins are partially transported across the mebrane
➢ sequence
•start transfer
•stop transfer (hydrophobic amino acids form transmembrane domain (alpha-helix))
Vesicular transport
➢Membrane ‘bags’ ➢Carry proteins •within vesicle •in vesicle membrane ➢Relies on membrane budding and fusion ➢ER <>golgi ➢Golgi <>secretory vesicles ➢Golgi/vesicles <> cell surface
➢Necessary for protein export
Vesicular transport: What does it rely on?
budding and fusion
•how we eat and secrete
➢Proteins on lumenal side remain lumenal
Vesicular transport: Vesicles leave ER
- Golgi (cisternae ↔ medial ↔ trans)
- cell membrane/secretion
- Transport to other endosomes
Vesicular transport: secretions and soluble proteins
➢Secretion
•Constitutive
•Regulated
➢Transmembrane proteins stay in the membrane
➢Soluble proteins get secreted into the extracellular space
What are the 3 main methods of protein transport?
- transport through nuclear pores
- transport across a membrane
- vesicular transport
Which amino acid forms signal sequences?
Import into nucleus: positively charged amino acids
export from nucleus: hydrophobic amino acid
Import into ER: lots of hydrophobic amino acids
What happens if you change the signal sequence?
Alter the localisation of the protein
What are the 3 methods of protein transport?
- Transport through nuclear pores
- Transport across membranes
- Transport by vesicles
Protein transport: between cytosol and nucleus
Gated transport
Protein transport: between cytosol and mitochondria, ER,peroxisomes, plastids
Transmembrane transport
Protein transport: cytosol and Golgi, Secretory vesicles lysosomes early/late endosome, cell surface
Vesicular transport
What is the cytosol topologically equivalent to?
Inside the nucleus
Are signal sequences always at the end of the protein?
No they can be anywhere along a protein, even distributed in different patches to form a signal when folded
Can a protein have more than one signal sequence?
Yes
It can have lots of different signal sequences depending on where it needs to go
Can gated transport specifically cytosol-nucleus go both ways?
Yes
Works because they are topologically equivalent
E.g. for transport of ribosomal subunits
