ER Translocation and Mitochondrial Import

Core Explanation

Part I: ER Translocation

Co-Translational Translocation: The SRP Cycle

Secretory and membrane proteins are targeted to the ER while still being synthesized. The key players:

The signal peptide is typically 20–30 amino acids at the N-terminus, organized as: a positively charged N-terminal region (n-region), a hydrophobic core of 7–15 residues (h-region), and a polar C-terminal region (c-region) containing the signal peptidase cleavage site. The hydrophobic core is the critical recognition element.

The SRP (signal recognition particle) is a conserved ribonucleoprotein composed of six polypeptides and a 300-nucleotide 7SL RNA. The SRP54 subunit contains a methionine-rich M-domain that binds directly to the hydrophobic core of the signal peptide as it emerges from the ribosomal exit tunnel (~40 residues from the peptidyl transferase center). SRP binding induces a translation elongation arrest (or pause) mediated by the Alu domain of the SRP RNA, which occupies the elongation factor binding site on the ribosome.

The SRP receptor (SR) is a heterodimeric GTPase (SRα + SRβ) anchored in the ER membrane. Interaction between SRP and SRα (both GTP-bound) delivers the ribosome–nascent chain complex to the Sec61 translocon. Mutual GTP hydrolysis releases SRP and SR, allowing translation to resume with the nascent chain now entering the Sec61 pore.

The Sec61 Translocon

Sec61 is a heterotrimeric complex (α, β, γ subunits) forming an hourglass-shaped pore. In the closed state, a short helical plug occludes the channel. Signal peptide binding displaces the plug, and the hydrophobic signal peptide intercalates between transmembrane helices of Sec61α, opening the pore laterally to the lipid bilayer (the "lateral gate"). Once the signal peptide is cleaved by signal peptidase, the mature N-terminus is free in the ER lumen. The elongating polypeptide threads through the pore as a disordered chain until folding begins in the lumen.

Generating Membrane Protein Topology

Soluble secretory proteins pass entirely through Sec61, with the signal peptide cleaved. Membrane proteins use additional signals:

• Signal-anchor sequence (type II): An internal, uncleaved hydrophobic sequence that enters Sec61 and is released laterally into the lipid bilayer. Translation continues, producing a single-pass membrane protein. The orientation depends on the distribution of positive charges flanking the anchor — the positive-inside rule: positively charged residues are more abundant on the cytosolic face.
• Stop-transfer sequence: A hydrophobic segment that halts translocation. After lateral release, translation continues in the cytosol, yielding a single-pass protein with one transmembrane domain and a cytosolic C-terminus.
• Multi-pass topology: Alternating signal-anchor and stop-transfer sequences, each released laterally in opposite orientations, produce polytopic proteins with multiple transmembrane helices (e.g., GPCRs with 7 transmembrane domains, glucose transporters with 12).

ER Folding and Modification

Chaperones: BiP (the ER Hsp70 homolog) binds exposed hydrophobic patches on nascent chains, preventing aggregation and providing a folding-competent environment. BiP's ATPase cycle is regulated by co-chaperones (J-domain proteins, nucleotide exchange factors). Calnexin and calreticulin are lectin chaperones that bind monoglucosylated N-linked glycans and retain glycoproteins until they are properly folded.

Disulfide bond formation: Protein disulfide isomerase (PDI) catalyzes the oxidation of cysteine pairs, forming stabilizing disulfide bonds. The oxidizing environment of the ER lumen (high GSSG:GSH ratio, maintained by Ero1) favors disulfide formation, whereas the reducing cytosol keeps most cysteines as free thiols.

N-linked glycosylation: The oligosaccharyltransferase (OST) complex transfers a preassembled 14-sugar core glycan (Glc₃Man₉GlcNAc₂) from a dolichol lipid carrier onto asparagine residues within the consensus sequence Asn-X-Ser/Thr (X ≠ Pro). This occurs co-translationally as the nascent chain enters the ER lumen. Glycosylation aids folding, serves as a quality control marker, and is later remodeled in the Golgi.

ER Quality Control, ERAD, and the UPR

ER quality control ensures only correctly folded proteins exit the ER. The calnexin/calreticulin cycle is central: glucosidase I and II trim two glucoses from the N-linked glycan. If the protein is folded, glucosidase II removes the final glucose, releasing the protein from calnexin/calreticulin. If misfolded, UDP-glucose:glycoprotein glucosyltransferase (UGGT) re-glucosylates the glycan, re-engaging calnexin for another folding attempt. Persistent misfolding targets the protein for ERAD.

ERAD (ER-associated degradation): Misfolded proteins are recognized, retro-translocated (dislocated) through a channel (the HRD1 complex, a ubiquitin ligase) into the cytosol, polyubiquitinated, and degraded by the 26S proteasome. The AAA-ATPase p97/VCP provides the pulling force for extraction. This is a remarkable process: proteins move backward through the same translocon that imported them, then are destroyed.

The unfolded protein response (UPR): When misfolded protein load exceeds ERAD capacity, three ER transmembrane sensors activate:

Together these pathways expand ER folding capacity, reduce protein influx, and — if stress is irremediable — trigger apoptosis.

Part II: Mitochondrial Import

The Challenge

Mitochondria contain ~1,500 proteins, but only 13 are encoded by the mitochondrial genome (all components of the respiratory chain). The rest are nuclear-encoded, synthesized on cytosolic ribosomes, and imported post-translationally. These precursor proteins must cross one or both mitochondrial membranes in a largely unfolded state.

TOM and TIM Complexes

TOM (Translocase of the Outer Membrane):

• Tom40: The central β-barrel pore through which all precursor proteins pass.
• Tom20/Tom70: Import receptors. Tom20 recognizes N-terminal presequences with their amphipathic α-helical character. Tom70 recognizes internal targeting signals, particularly on hydrophobic membrane proteins (carrier proteins, metabolite transporters).
• Tom22: A central receptor that also stabilizes the complex.
• Tom5/6/7: Small subunits that assist assembly and translocation.

TIM23 (Translocase of the Inner Membrane):

• Tim23/Tim17: Form the inner membrane translocation channel.
• Tim44: Anchors matrix Hsp70 at the import site.
• PAM complex (Presequence translocase-Associated Motor): Matrix Hsp70 (mtHsp70/Ssc1) + Tim44 + co-chaperones (Pam18, Pam16, Mge1). mtHsp70 binds the incoming precursor in an ATP-dependent manner and, through conformational changes, acts as a Brownian ratchet to pull the polypeptide into the matrix.

TIM22: A distinct translocase that inserts hydrophobic inner membrane proteins (carrier family) into the inner membrane without passage into the matrix.

SAM (Sorting and Assembly Machinery): Inserts β-barrel proteins (e.g., porin/VDAC) into the outer membrane.

The Import Pathway

1. Cytosolic chaperoning: Hsp70 and Hsp90 chaperones maintain the precursor in an unfolded, import-competent state, preventing aggregation.
2. Recognition: Tom20/Tom70 bind the presequence or internal targeting signal.
3. TOM translocation: The precursor passes through Tom40. The N-terminal presequence enters the intermembrane space.
4. Membrane potential-dependent translocation: The presequence, enriched in positively charged and hydroxylated residues, is electrophoresed into the matrix across TIM23. The inner membrane potential (Δψ, ~180 mV, negative inside) is essential — it pulls the positively charged presequence through TIM23. Proteins lacking a presequence (inner membrane carriers) use TIM22 in a Δψ-dependent but presequence-independent manner.
5. Matrix processing: The mitochondrial processing peptidase (MPP) cleaves the presequence. mtHsp70, powered by ATP hydrolysis, completes import by binding and pulling the precursor into the matrix.
6. Folding: Mitochondrial Hsp60 (a GroEL homolog) and Hsp10 mediate folding in the matrix. For proteins of the inner membrane, specific chaperones guide insertion.

Energy Requirements

Mitochondrial import is energetically expensive:

• Δψ: Absolutely required for TIM23-mediated import. Dissipating the membrane potential with uncouplers (e.g., FCCP) blocks import instantly.
• Cytosolic ATP: Powers Hsp70/Hsp90 chaperones that maintain the precursor unfolded.
• Matrix ATP: Powers mtHsp70 (the import motor) and Hsp60 (folding).

Key Contrast: ER vs. Mitochondrial Import

Disease and Clinical Connections

• ER stress and diabetes: Pancreatic β-cells secrete massive amounts of insulin. Chronic ER stress from demand overload triggers PERK-mediated eIF2α phosphorylation, β-cell apoptosis, and diabetes progression.
• Wolfram syndrome: Mutations in WFS1 (wolframin), a transmembrane ER protein involved in the UPR, cause diabetes insipidus, diabetes mellitus, optic atrophy, and deafness (DIDMOAD).
• Charcot-Marie-Tooth disease type 2A: Mutations in mitofusin 2 (MFN2), a mitochondrial outer membrane fusion protein, impair mitochondrial dynamics.
• Leigh syndrome: Mutations in mitochondrial import components (e.g., TIMM8A/DDP1 causing deafness-dystonia syndrome) or respiratory chain subunits produce devastating neurodegeneration in infancy.
• Friedreich's ataxia: Frataxin, a mitochondrial matrix protein involved in iron-sulfur cluster assembly, has impaired import or function due to GAA repeat expansion.
• Cancer and the UPR: Tumor cells live under chronic ER stress (hypoxia, nutrient deprivation, high secretory load). They upregulate the UPR as a survival mechanism. IRE1 and PERK are therapeutic targets — blocking the UPR can selectively kill stressed cancer cells.

High-Yield Summary

• ER translocation: Co-translational. SRP → SRP receptor → Sec61. Signal peptide (cleaved) vs. signal-anchor (lateral release) vs. stop-transfer (halt).
• Membrane topology: Positive-inside rule. Alternating signal-anchor/stop-transfer yields multi-pass proteins.
• ER modifications: BiP chaperones, calnexin/calreticulin cycle (lectin-based quality control), PDI (disulfide bonds), OST (N-linked glycosylation).
• ER quality control: ERAD (HRD1 → ubiquitination → proteasome) and UPR (IRE1/XBP1, PERK/eIF2α/ATF4, ATF6).
• Mitochondrial import: Post-translational. TOM20/70/40 → TIM23 (presequence, Δψ-dependent) or TIM22 (carrier proteins). mtHsp70 = import motor.
• Δψ is essential: FCCP blocks mitochondrial import within seconds.
• ER vs. mitochondria: Different evolutionary origins → different import strategies, but both use signal peptides, translocons, and ATP/GTP-driven steps.

Practice Questions

1. A gene encoding a cytosolic enzyme is engineered to include an N-terminal ER signal peptide. Predict the localization and glycosylation status of the resulting protein. Now delete the signal peptide cleavage site. What changes?

Answer: With an intact signal peptide and cleavage site, the protein is co-translationally translocated into the ER lumen. It will be N-glycosylated at any Asn-X-Ser/Thr sequons. Signal peptidase cleaves the signal peptide. The mature protein then travels through the Golgi (glycan processing) and is secreted if it lacks retention signals. If the cleavage site is deleted, the signal peptide is not cleaved. The hydrophobic signal peptide remains as an N-terminal extension. The unprocessed protein may still be secreted, but the hydrophobic tail could cause aggregation, ER retention by quality control, or aberrant membrane association. Function is likely impaired.

2. You treat cells with the protonophore FCCP, which dissipates the mitochondrial inner membrane potential. Which mitochondrial proteins continue to be imported and which are blocked? Explain.

Answer: Proteins with N-terminal presequences targeted to the matrix or inner membrane via TIM23 are blocked — the positively charged presequence requires the electrophoretic driving force of Δψ (negative matrix side) for translocation across the inner membrane. Inner membrane carrier proteins with internal targeting signals imported via TIM22 are also blocked because TIM22-mediated insertion is Δψ-dependent. However, proteins destined only for the mitochondrial outer membrane (e.g., porin/VDAC via the TOM complex and SAM complex) are not blocked by FCCP, because TOM translocation does not require Δψ, and SAM-mediated β-barrel insertion occurs at the outer membrane. Similarly, intermembrane space proteins that use the MIA pathway (oxidative folding) are TOM-dependent but do not require Δψ for the MIA step. Key principle: Δψ is required for proteins that must cross or insert into the inner membrane.

3. A cell line expresses a mutant BiP that cannot hydrolyze ATP. What is the consequence for (a) protein folding in the ER and (b) ERAD?

Answer: (a) BiP binds hydrophobic patches on nascent chains; ATP hydrolysis locks BiP onto the substrate (high-affinity state), and nucleotide exchange (ATP for ADP) releases BiP. A BiP mutant that cannot hydrolyze ATP remains in the low-affinity, ATP-bound state. It cannot stably bind unfolded proteins, so chaperone function is lost. Nascent chains aggregate. (b) ERAD is also impaired. BiP is involved in recognizing misfolded proteins for retrotranslocation — it binds exposed hydrophobic patches and helps feed the substrate into the dislocation channel. Moreover, chronic accumulation of misfolded proteins hyperactivates the UPR, which if unresolved triggers apoptosis.

Common Misconceptions

"All proteins unfold to enter the ER." No. ER translocation is co-translational — the protein never exists as a fully folded species in the cytosol. It threads into the ER as an extended nascent chain. Mitochondrial precursors, however, must be kept unfolded or partially unfolded by cytosolic chaperones and refold in the matrix.

"ERAD is the same as autophagy." No. ERAD targets individual misfolded proteins to the cytosolic 26S proteasome. Bulk ER degradation through autophagy (ER-phagy) is a separate pathway. ERAD handles soluble/membrane misfolded proteins one at a time.

"The UPR is always a bad thing." No. The UPR is adaptive at low-to-moderate levels — it expands ER capacity and restores homeostasis. Only chronic, unresolvable ER stress triggers the apoptotic branch (CHOP). The UPR is essential for normal physiology of professional secretory cells (plasma cells, pancreatic acinar cells, hepatocytes).

"The mitochondrial membrane potential only powers ATP synthesis." No. Δψ is also absolutely required for mitochondrial protein import (electrophoretic presequence translocation through TIM23), ion transport, and metabolite exchange. The import function of Δψ likely predates its role in oxidative phosphorylation — it may be the most ancient function of the mitochondrial membrane potential.