Module 1 · The Entry Gate

Cotranslational Translocation & the Sec61 Channel

Günter Blobel’s 1975 signal hypothesis proposed that newly synthesised secretory proteins carry an N-terminal “address” sequence directing them to the ER. He shared the 1999 Nobel Prize for this work. The signal peptide is recognised by the signal recognition particle (SRP), which delivers the ribosome to the ER membrane, where a protein-conducting channel — the Sec61 translocon — threads the nascent chain into the lumen. Tom Rapoport’s structural biology over the following three decades elucidated the translocon’s mechanism at atomic resolution.

1. The Signal Peptide

A signal peptide is a short (16–30 residue) N-terminal stretch with three features:

N-region: 1–5 residues, often positively charged;
H-region: 7–15 residues, hydrophobic (often leucine/alanine-rich, can adopt a helix);
C-region: 3–7 residues, polar, ending in a signal peptidase cleavage site (Ala-X-Ala consensus).

Signal peptides are cleaved by the luminal signal peptidase complex during insertion. Membrane proteins use similar hydrophobic stretches as signal anchors that are not cleaved but remain embedded as the first transmembrane segment.

2. SRP & the SR Cycle

The signal recognition particle (SRP) is a 325 kDa ribonucleoprotein complex: 7SL RNA + six proteins (SRP9/14, SRP19, SRP54, SRP68/72). SRP54 carries the signal-binding pocket (a methionine-rich hydrophobic groove) and a GTPase domain. SRP9/14 contact the ribosome near the exit tunnel and cause elongation arrest upon signal recognition.

The SRP cycle:

Signal peptide emerges from ribosome exit tunnel; SRP binds.
Elongation arrests (SRP9/14 alarmone).
SRP-ribosome complex targets to the ER via SRP receptor (SR), a heterodimeric GTPase (SRαβ) anchored in the ER membrane.
SR and SRP54 reciprocally stimulate GTP hydrolysis; signal handed off to Sec61.
SRP dissociates; elongation resumes; protein threads into the translocon.

The GTPase cycle doubles as a proofreading mechanism: if SR fails to engage quickly, GTP hydrolysis resets SRP and releases the ribosome — preventing aberrant translocation of non-signal-bearing proteins.

3. The Sec61 Translocon

The eukaryotic Sec61 complex is heterotrimeric: Sec61α (10 TM helices, the channel), Sec61β (1 TM, accessory), Sec61γ (1 TM, structural). Archaeal SecY/E/G is the homologue. The first atomic structure (van den Berg 2004) revealed key features:

Hourglass channel: narrow pore ring of six hydrophobic residues at the centre, which seal around a translocating polypeptide.
Plug helix (TM2a): occupies the channel in the closed state, displaced when a substrate enters.
Lateral gate: a helical cleft between TM2b and TM7 that opens sideways into the lipid bilayer — the route by which transmembrane segments of integral membrane proteins exit into the bilayer during insertion.

Cryo-EM of ribosome-bound Sec61 (Voorhees 2014, 2016) showed how hydrophobic segments of the nascent chain partition laterally based on their transfer-free-energy into the lipid phase (Hessa 2007 “biological hydrophobicity scale”). Partitioning is near-equilibrium: the lateral gate is the threshold at which hydrophobicity decides whether a stretch enters the lumen or stays in the membrane.

Simulation: Hydropathy-Based Topology Prediction

Python

script.py51 lines

import numpy as np, matplotlib
matplotlib.use('Agg')
import matplotlib.pyplot as plt

# Kyte-Doolittle hydropathy scan of a model signal peptide + membrane protein
# High hydrophobicity at N-terminus (signal peptide, ~20 aa)
# Second TM segment around position 150

np.random.seed(42)
# Kyte-Doolittle hydropathy (arbitrary units)
n_residues = 300
hydropathy = np.random.normal(0, 1, n_residues)
# Insert signal peptide (high hydropathy)
hydropathy[5:25] += 3.0
# Insert a TM segment
hydropathy[150:170] += 3.5

# Smoothing window
def kd_smooth(x, w=7):
    return np.convolve(x, np.ones(w)/w, mode='same')
h_smooth = kd_smooth(hydropathy, w=9)

fig, ax = plt.subplots(figsize=(12, 5), facecolor='#0a0a1a')
ax.set_facecolor('#111827'); ax.tick_params(colors='#cbd5e1')
for s in ax.spines.values(): s.set_color('#334155')
ax.grid(True, color='#334155', alpha=0.3)

ax.fill_between(range(n_residues), h_smooth, 0, where=(h_smooth>1.5),
                color='#34d399', alpha=0.5, label='Predicted TM / signal (h>1.5)')
ax.plot(range(n_residues), h_smooth, color='#fbbf24', lw=1.8)
ax.axhline(1.5, color='#f87171', ls='--', alpha=0.7, label='TM threshold')
ax.axhline(0, color='#94a3b8', lw=0.5)

# Label
ax.annotate('Signal peptide\n(cleaved after insertion)', xy=(15, 3), xytext=(50, 3.5),
            color='#fde68a', fontsize=9,
            arrowprops=dict(arrowstyle='->', color='#fde68a'))
ax.annotate('TM segment\n(lateral gate exit)', xy=(160, 3.5), xytext=(200, 3.5),
            color='#fde68a', fontsize=9,
            arrowprops=dict(arrowstyle='->', color='#fde68a'))

ax.set_xlabel('Residue position', color='#cbd5e1')
ax.set_ylabel('Hydropathy (Kyte-Doolittle, smoothed)', color='#cbd5e1')
ax.set_title('Signal/topology prediction from hydropathy',
             color='#6ee7b7', fontweight='bold')
ax.legend(facecolor='#1e293b', edgecolor='#334155', labelcolor='#cbd5e1')

plt.tight_layout()
plt.savefig('output.png', dpi=120, bbox_inches='tight', facecolor='#0a0a1a')
print('Kyte-Doolittle 1982: hydropathy scan identifies signal peptides and TM helices')
print('von Heijne positive-inside rule: positive charges flank cytoplasmic face')

Click Run to execute the Python code

Code will be executed with Python 3 on the server

4. The Positive-Inside Rule

Gunnar von Heijne’s 1986 observation: cytoplasmic loops of integral membrane proteins are consistently enriched in positively charged residues (Arg/Lys) compared with luminal/extracellular loops. This positive-inside rule is the single strongest topology-determining signal: charge imbalance flanking the first transmembrane segment dictates its orientation in the translocon, and subsequent TMs follow obligate alternation.

The underlying mechanism is electrostatic: the Sec61 lateral gate environment disfavours positive charges on the luminal side. The rule is strong enough that transplanting a positive patch from the cytoplasmic to the luminal face of a TM helix can invert the topology of the entire protein. It makes membrane topology predictable from sequence with ~95% accuracy by modern algorithms (DeepTMHMM, 2022).

5. Post-Translational Insertion: GET/TRC40, EMC, PEX

Not all ER proteins enter cotranslationally:

Tail-anchored (TA) proteins: single C-terminal TM domain emerges only after release from the ribosome, too late for SRP. The GET pathway (yeast) / TRC40 pathway (mammals) delivers them. Sgt2/SGTA chaperone hands them to the Get3/TRC40 ATPase, which docks on the Get1/Get2/WRB-CAML receptor for release into the membrane.
EMC (ER membrane protein complex): discovered by Ng/Hegde (2014, 2018) as a post-translational insertase for polytopic membrane proteins with moderately hydrophobic TM1. Can work both cotranslationally alongside Sec61 and post-translationally for rhodopsin, SLC transporters, and GPCRs.
Peroxisomal matrix proteins: insertion by PEX5/PEX7-mediated pathway (Module 0 of the Peroxisome course).

The field has moved from a single-translocon view to a translocon network (Hegde & Keenan 2022) with substrate-specific insertases.

6. Accessories at the Translocon

The Sec61 channel rarely works alone. Core accessories: the Sec62/Sec63 complex (post-translational support, BiP recruitment), TRAP(translocon-associated protein, promotes signal-peptide gating in weak substrates), the oligosaccharyltransferase (OST/STT3)that glycosylates nascent Asn-X-Ser/Thr motifs as they enter the lumen, the signal peptidase complex, and luminal BiP acting as a molecular ratchet (Brownian ratchet model of Matlack & Rapoport 1998). The result is a translocation machine that simultaneously threads, glycosylates, cleaves the signal, and initiates folding on a second-to-minute timescale per protein.

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