Protein trafficking

forwards ER to golgi to plasma membrane

eg golgi to ER

cell exterior to early to late endosome

vesicle buds from donor compartment, pinches off and translocates to acceptor compartment. this is a scission event. vesicle docks with acceptor membrane; targeting event. vesicle fuses with acceptor compartment and releases its contents into the lumen

coat proteins bind to specific sequences on cytoplasmic tail of transmembrane proteins acting as receptors. a cluster forms due to the affinity of coat proteins to eachother and the receptors. cargo is selectively recruited into buds as they form.

stalk-like structure scission occurs by dynamin (GTPase) that constricts its neck clathrin coated vesicle detaches from membrane then uncoats and the naked vesicle fuses with target

triskelion trimer: 3 heavy chain subunits and 3 light chain subunits assemble into cages via protein-protein interactions and deform membrane into a sphere

from the golgi apparatus and plasma membrane differences in adaptor proteins involved

COPI

COPII

clathrin and adaptin 1 golgi body lysosome, via endosomes

clathrin and adaptin 2 plasma membrane endosomes

COP proteins ER, golgi cisternae, golgi apparatus golgi apparatus, golgi cisternae, ER

some molecules are passively included in vesicles (not selected)

active recruitment selective exclusion passive inclusion

coat recruitment GTPases

Sar1 is a small GTPase that is initially in its inactive, soluble form bound to GDP (Sar1-GDP) in the cytosol. Sar1-GDP interacts with a guanine nucleotide exchange factor (Sar1-GEF) located on the ER membrane. This interaction facilitates the exchange of GDP for GTP, converting Sar1 to its active form (Sar1-GTP). The active Sar1-GTP exposes an amphiphilic helix that allows it to insert into the ER membrane.

Sar1-GTP recruits coat protein complex II (COPII) subunits to the ER membrane. COPII subunits include proteins such as Sec23/24. Sec23 has an affinity for the Sar1 protein, which is anchored in the ER membrane in its active form (Sar1-GTP). Sec24, another component, binds to specific tails on cargo receptors or integral membrane proteins that are destined to leave the ER. This ensures that the right cargo is selected for transport. Recruitment of Outer Coat Proteins: Outer coat proteins, such as Sec13/31, are recruited because of their affinity for the inner coat proteins (Sec23/24). This interaction begins to deform the ER membrane, forming a vesicle bud. Selective Recruitment by Integral Membrane Proteins: The integral membrane proteins that are bound to the coat help selectively recruit the coat to ensure specific cargo is included in the vesicle. Cargo receptors have a specific affinity for cargo molecules, guiding them into the lumen of the budding vesicle

hydrolysis of GTP on Sar 1 conformational change retracts amphipathic helix loses affinity for inner coat inner coat drops off, followed by outer coat

on cargo receptor binds to sec 24 D=aspartic X=any E=glutamic diacidic motif

by chaperones, BIP keeps proteins in ER lumen eg antibodies until correctly assembled: BIP binds to heavy chain, retaining unassembled antibody in ER. it cannot bind to a fully assembled antibody

ER resident proteins have a KDEL sequence at their C-termini (lysine, aspartic acid, glutamic acid, leucine). these are retrieved in COPI vesicles

KDEL receptors (transmembrane protein) selectively packaged into COPI coated vesicle in ER, KDEL receptor released KDEL containing protein and is returned to golgi via COPII vesicles.

Rab proteins (small GTP binding proteins) SNARE proteins give vesicles and target membranes a molecular identity