#retrograde transport

Protein Sorting and Trafficking

Which protein mediates retrograde transport of ER proteins from the Golgi?

Maintaining ER proteome against continuous leakage via bulk flow into Golgi relies active retrieval preventing depletion resident chaperones folding enzymes. ER membrane proteins expose C-terminal dilysine motif KKXX or KXKXX with two lysines at -3 -4 positions relative extreme C-terminus recognized directly WD40 beta-propeller domains COPI subunits alpha-COP beta prime-COP binding dilysine pH independent. Soluble ER luminal residents like BiP HSPA5 GRP94 PDI calreticulin bear C-terminal KDEL; after escape bind KDEL receptor family Erd2 homologs KDELR1-3 seven transmembrane proteins concentrated cis-Golgi ERGIC mildly acidic pH favors high-affinity binding. Receptor-cargo complex displays cytosolic dilysine-like retrieval signal receptor tail directly interacts COPI coatomer causing concentration COPI coated vesicles budding Golgi rims. COPI assembly triggered small GTPase ARF1-GTP produced GBF1 GEF generating membrane curvature. Vesicles move retrogradely ERGIC ER where de-coating SNARE-mediated fusion via syntaxin18 Use1 BNIP1 Sec22b complex returns content ER lumen membrane. Upon ER arrival neutral pH causes KDEL release. COPII Sec23-24 performs anterograde export, clathrin mediates TGN-endosome trafficking endocytosis dynamin performs scission. Brefeldin A pharmacologically inhibits ARF GEF blocking COPI formation demonstrating indispensability retrograde ER protein return Golgi ER quality control and homeostasis.

Ref: Bethune et al., Cold Spring Harb Perspect Biol 2006: COPI retrieval of ER proteins from Golgi.

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Protein Sorting and Trafficking

Which coat protein mediates retrograde transport from the Golgi to ER?

Retrieval pathways restoring escaped ER proteins and itinerant SNAREs rely on COPI coat protein complex I heptamer. COPI subunits alpha, beta, beta prime, gamma, delta, epsilon, zeta assemble 600 kDa coatomer. At Golgi cisternae and ERGIC tubules ARF1-GTP generated by GBF1 GEF recruits en bloc coatomer via gamma-beta-delta trunk interaction. Membrane deformation creates 50-60 nm vesicles containing cargo exposing KKXX or KXKXX dilysine motif at C-terminus binding WD40 propeller of alpha and beta prime COP subunits and KDEL receptor-cargo complexes where receptor tail contains KKXX-like signals. COPII comprising Sec23-24 and Sec13-31 does outward ER to Golgi leg recognizing di-acidic motifs, while clathrin plus AP1 AP2 GGA serves trans-Golgi-to-endosome and plasma-membrane-to-endosome routes sorting via tyrosine and dileucine. AP3 can function with clathrin or as non-clathrin coat for lysosomal membrane proteins. Pharmacologic blockade brefeldin A inhibits ARF GEF collapsing Golgi into ER within minutes by blocking COPI assembly. Hence COPI dedicated retrograde Golgi-to-ER carrier essential for ER proteostasis recycling SNAREs and maintaining Golgi enzyme gradients during cisternal maturation and homeostasis.

Ref: Alberts et al., MBC: COPI coat mediates retrograde Golgi-to-ER transport and retrieval.

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Protein Sorting and Trafficking

How does the KDEL receptor regulate protein retrieval?

ER lumen contains chaperones BiP HSPA5, Grp94 HSP90B1, PDI family, calreticulin that may leak into Golgi during non-selective bulk flow; retention therefore relies on active retrieval not static retention. Soluble residents carry C-terminal KDEL tetrapeptide Lys-Asp-Glu-Leu or variant HDEL. If they escape to cis-Golgi beyond, KDEL receptors family of seven-transmembrane proteins Erd2 in yeast KDELR1-3 in mammals concentrated in Golgi and ERGIC recognize tetrapeptide. Binding strongly pH dependent because histidine residues protonate at mildly acidic Golgi lumen pH 6.0-6.2 stabilizing interaction with C-terminus, while neutral ER lumen pH 7.2-7.4 causes proton release dramatic loss affinity causing dissociation. Ligand-bound receptors packaged into COPI vesicles for retrograde return via dilysine-like signals on receptor tails binding coatomer. Upon reaching ER cargo dissociates spontaneously without covalent modification. Free receptors recycle forward via COPII. pH sensor mechanism ensures unidirectional net return without phosphorylation or SNARE inhibition, explaining why neutralization of Golgi pH causes ER protein secretion.

Ref: Alberts et al., MBC Chapter 13: KDEL receptor pH-dependent retrieval to ER via COPI.

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Protein Sorting and Trafficking

In the vesicle transport model, how are Golgi-resident proteins returned to their compartments?

Golgi organization is explained by cisternal maturation model where new cis cisternae arise from fusion of ER-derived COPII vesicles at ERGIC and progressively mature into trans cisternae carrying secretory cargo forward toward trans-Golgi network. Resident processing enzymes, including cis mannosidases, medial GlcNAc transferases and trans sialyltransferases, cannot move forward with cargo otherwise compartmental polarity would be lost and glycosylation random. To preserve sequential enzyme gradients, transmembrane residents are selectively extracted into COPI-coated vesicles that transport retrogradely to younger cisternae. Cytosolic tails expose signals recognized by adaptors such as Vps74, GOLPH3 and COG complex concentrating enzymes into COPI buds. Live imaging in yeast shows COPI vesicles carrying medial mannosidase II backward while cargo remains lumenal and anterograde carriers move forward. Anterograde vesicle shuttle models propose stable cisternae, but COPI mutants and Rab effectors and tether assays demonstrate retrograde return essential for maturation, not degradation, lateral diffusion, or default secretion pathway maintaining fidelity and polarity.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 13: Cisternal maturation and COPI retrograde Golgi traffic.

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