Module 1 · Acidification

V-ATPase & Acidification

The lysosome maintains an internal pH of 4.5–5.0 against a cytosolic pH of ~7.2: a ~1000-fold H⁺ gradient. The vacuolar H⁺-ATPase (V-ATPase)is the pump that establishes and maintains this gradient. It is an elaborate rotary motor, structurally homologous to the mitochondrial F₁F₀ ATP synthase but running in reverse: consuming ATP to pump protons uphill.

1. V-ATPase Structure

V-ATPase is a 900 kDa rotary enzyme in two subcomplexes:

V₁: cytosolic head, eight subunits (A₃B₃ catalytic hexamer + C + D + E + F + G + H). Hydrolyses ATP.
V₀: membrane-embedded proton channel, ~10 subunits including a c-ring of 10 c-subunits, plus a, d, e, and accessory proteins.

The rotary mechanism mirrors F₁F₀: ATP hydrolysis in V₁drives rotation of the central stalk, which turns the c-ring, which carries protons across the membrane. H⁺/ATP ratio is ~2 (10 c-subunits / 3 ATP-hydrolysis events × ring rotation). Cryo-EM structures (Mazhab-Jafari 2016, Kishikawa 2022, Wang 2020) have resolved multiple conformational states.

2. Regulation by V₁-V₀ Assembly

Unique to V-ATPase: V₁ and V₀ can reversibly dissociate under glucose starvation (Kane 1995), shutting off pump activity when nutrients are low. Assembly is controlled by the RAVE complex in yeast; in mammals, by the mTORC1 pathway at the lysosomal surface (Module 5 of this course). This coupling means lysosomal acidification is itself sensitive to cellular nutrient state — a feedback loop that links proteostasis to energy.

V-ATPase proton-pumping activity varies even within a single lysosome population; newly-formed lysosomes have lower activity and higher pH, maturing over hours to full acidity.

3. Counter-Ion: ClC-7 / OSTM1

Pumping H⁺ alone would build a large membrane potential (Δψ), quickly stopping the pump. A counter-ion must flow. ClC-7 (a chloride/proton antiporter; 2 Cl⁻ in for 1 H⁺ out) handles this, dissipating the chemical but not the pH gradient. ClC-7 + its obligate partner OSTM1 are dedicated lysosomal antiporters. Loss-of-function causes infantile malignant osteopetrosis (OSTM1, CLCN7 — osteoclast failure because osteoclast resorption depends on lysosomal acidification to digest bone matrix) and a lysosomal neurodegeneration in double-knockout mice.

Simulation: pH Set-Point from Pump-Leak Balance

Python

script.py35 lines

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

# V-ATPase pH setpoint: pump vs leak balance
# pH_ss = pH_cyto - log10(J_pump/J_leak)

ratio = np.logspace(-1, 4, 300)
pH_cyto = 7.2
pH_lumen = pH_cyto - np.log10(ratio)

fig, ax = plt.subplots(figsize=(11, 6), 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.semilogx(ratio, pH_lumen, color='#d946ef', lw=2.6)
# Compartment markers
compartments = [('Early endosome', 6.3), ('Late endosome', 5.5),
                ('Lysosome', 4.7), ('Secretory granule', 5.0)]
colors_comp = ['#2dd4bf', '#fbbf24', '#f87171', '#60a5fa']
for (name, pH), c in zip(compartments, colors_comp):
    ax.axhline(pH, color=c, ls='--', alpha=0.5, label=f'{name} pH {pH}')

ax.set_xlabel('J_pump / J_leak ratio', color='#cbd5e1')
ax.set_ylabel('Steady-state luminal pH', color='#cbd5e1')
ax.set_title('V-ATPase set-point: pump-leak balance',
             color='#f0abfc', fontweight='bold')
ax.legend(facecolor='#1e293b', edgecolor='#334155', labelcolor='#cbd5e1')
ax.invert_yaxis()

plt.tight_layout()
plt.savefig('output.png', dpi=120, bbox_inches='tight', facecolor='#0a0a1a')
print('Lysosomal pH 4.7 requires J_pump/J_leak ~ 300x over cytosol')
print('Regulated by V-ATPase assembly, Cl- counter-ion flux (ClC-7)')

Click Run to execute the Python code

Code will be executed with Python 3 on the server

4. Chloroquine, Bafilomycin, and Other Inhibitors

Lysosomal pH can be manipulated:

Bafilomycin A1: binds the c-ring of V₀, inhibiting rotation. The classical V-ATPase inhibitor experimentally.
Chloroquine / hydroxychloroquine: weak bases that accumulate in acidic compartments and neutralise them. Used clinically in malaria (inhibits haem crystallisation in Plasmodium digestive vacuole), RA, lupus; notable experimental autophagy inhibitor. COVID-19 trials failed to show benefit.
Ammonium chloride: freely diffusing weak base; simple experimental deacidification.
Saliphenylhalamide, concanamycin: research-tool V-ATPase inhibitors.

V-ATPase inhibition deacidifies lysosomes, inactivates hydrolases, blocks autophagy-lysosome fusion, and — in many tumours — selectively kills autophagy-dependent cancer cells. Clinical development of V-ATPase inhibitors in oncology is ongoing.

5. Tissue-Specific V-ATPase Isoforms

Several V-ATPase subunits have tissue-specific isoforms: V₀-a3 in osteoclasts (TCIRG1; loss causes osteopetrosis); V₀-a4 in kidney α-intercalated cells (loss causes distal renal tubular acidosis). These isoforms tune V-ATPase for specific physiological roles — acid secretion, rather than lysosomal acidification.

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