Secretory Pathways

Question 1

Mechanisms of protein transport (3)

Answer 1

Gated transport between cytosol and nucleus via barrier Nups and Karyopherins

Transmembrane transport between cytosol and organelle lumens via translocator proteins

Vesicular transport between cellular compartments

Question 2

Major functions of ER (6)

Answer 2

Lipid synthesis; cholesterol regulation; calcium storage (smooth); protein synthesis and folding; protein quality control; insertion of proteins into membrane

Question 3

Co-Translational Translocation

Answer 3

Pool of ribosome subunits in cytoplasm associate with mRNA and begin translation. Signal sequence on nascent polypeptide (NPP) is recognized by P54 subunit of signal recognition particle (SRP; folded RNA and associated proteins). Translation stops, SRP brings ribosome to SRP-R on ER which lines NPP up with translocon. Translation continues with NPP entering ER. Signal peptidase removes signal and leaves it in the ER mmbr. BiP chaperone helps fold NPP as it enters.

Question 4

Type I Transmembrane protein synthesis

Answer 4

Have signal sequence at amino terminus. Fed into translocon, conserved stop transfer sequence stops translocation so transmembrane domain is left in ER membrane and rest of protein is synthesized in cytosol.

Question 5

Type II Transmembrane protein synthesis

Answer 5

Protein with carboxy terminus in ER lumen. Signal sequence is upstream of amino terminus so protein is partially translated before translocation to ER. The remainder of protein is fed into ER so that amino terminus is cytosolic (thanks to positive charges) and carboxy terminus in extracellular. Signal sequence is transmembrane domain

Question 6

Multiple Transmembrane domains

Answer 6

proteins with alternating stop and start translocating domains. (Start are likely signal sequences similar to type II that get fed into translocon and become anchoring domains?)

Question 7

Major functions of Golgi (4)

Answer 7

Synthesis of sphingolipids from ceramide
Protein modifications: glycosylation and sulfide addition
Proteolytic processing
Localization of proteins within cell

Question 8

Coat Proteins (general mechanism)

Answer 8

Assist with budding and recognize localization signals for intracellular targeting of proteins.
Cargo receptors bind proteins, adaptin proteins recognize motifs on cargo Rs or transmembrane proteins for localization. Adaptins associate with coat protein for vesicle formation.

Question 9

Types of coat proteins

Answer 9

Clathrin: Moves from Golgi to PM, PM to Golgi, PM to other locations
COPII: moves from ER to Golgi, associates with Kinesin moving negative to positive down microtubules
COPI: moves from Golgi to ER, associates with Dynein moving positive to negative down microtubules