Western Blot — Cysteine Proteinases
Cysteine proteinases (cathepsins B, C, F, H, K, L, O, S, V, W, X/Z, and the calpain family) share the catalytic cysteine mechanism that makes them autoactivate, cleave each other, and require specific inhibitor cocktails in the lysis buffer. This protocol covers the specific adjustments to routine Western blot for cysteine proteinase targets.
Browse cysteine proteinase antibodiesCysteine proteinases in humans fall into two structurally distinct families: the papain-like cathepsins (family C1) which are lysosomal or secreted enzymes with a conserved Cys-His-Asn catalytic triad, and the calpains (family C2) which are cytosolic calcium-dependent proteinases with a distinct fold. The two families do not cross-react at the antibody level, but they share the same core biochemical challenge for Western blot analysis: they autoactivate under permissive conditions and they cleave themselves and their neighbours during sample preparation.
This protocol covers Western blot detection of cysteine proteinases with Triple Point rabbit polyclonal antibodies. Standard Western blot mechanics are covered in dilution optimization and controls and validation; this document focuses on cysteine-proteinase-specific considerations.
Sample preparation — the autoactivation problem
Cathepsins autoactivate at acidic pH (pH 4-5). Calpains autoactivate at elevated calcium (millimolar Ca2+) with limited proteolysis of their own propeptide. Both processes happen during cell lysis if you do not actively suppress them:
Neutral-pH lysis with cysteine inhibitor
Standard for detecting the population of forms as they exist in the cell. Buffer: 50 mM Tris-HCl pH 7.4, 150 mM NaCl, 1% Triton X-100, complete protease inhibitor cocktail plus E-64 (5-10 μM) as a specific cysteine proteinase inhibitor. E-64 (trans-Epoxysuccinyl-L-leucylamido(4-guanidino)butane) is cell-permeable and irreversibly inhibits all papain-family cysteine proteinases at low micromolar concentrations. Effective against cathepsins B, C, H, K, L, S, V, X.
Calcium-chelating lysis for calpains
For calpain detection specifically, add 5 mM EDTA or 1 mM EGTA to the lysis buffer to chelate calcium and prevent calpain autoactivation. Calpains are inactive without calcium; chelation freezes them in the pre-activation state. This is not compatible with metalloproteinase co-detection on the same lysate — run separate lysates for calpains vs MMPs.
Denaturing lysis (single-shot for Western blot only)
Preferred for pure Western blot analysis. Boil cells directly in Laemmli sample buffer — 100 μL 2× buffer added to washed cell pellet, immediate 95°C for 10 minutes. Denatures the target, freezes activation state at harvest time, eliminates all autoactivation and cross-cleavage during processing.
Class-specific inhibitor recipe
Full cocktail: E-64 (5 μM), leupeptin (10 μM), pepstatin A (5 μM — blocks aspartic proteinases that could cross-cleave cathepsins), AEBSF (100 μM — blocks serine proteinases), plus 1× complete protease inhibitor cocktail. This cocktail preserves cysteine proteinase population without cross-cleavage from other classes.
Expected molecular weights — procathepsin, single-chain, two-chain
Cathepsins have a distinctive multi-form pattern that is diagnostic when interpreted correctly:
Cathepsin B
Procathepsin B: ~46 kDa. Single-chain mature: ~33 kDa. Two-chain mature: ~25 kDa heavy chain + ~5 kDa light chain (light chain often runs off gel). Most cell lines show a mix of pro and single-chain, with two-chain enriched in late lysosomes.
Cathepsin L
Procathepsin L: ~41 kDa. Single-chain mature: ~30 kDa. Two-chain mature: ~25 kDa heavy chain + ~5 kDa light chain. Similar mixed-form pattern to cathepsin B in lysosomes.
Cathepsin D-like C1 family members
Cathepsin S: ~37 kDa proenzyme, ~28 kDa mature. Cathepsin K: ~37 kDa proenzyme, ~29 kDa mature. Cathepsin C: ~55 kDa proenzyme, exists as a tetramer of heavy + light chain dimers, complex band pattern under reducing conditions.
Calpains
Calpain-1 (μ-calpain): ~80 kDa catalytic subunit + ~28 kDa regulatory subunit. Calpain-2 (m-calpain): ~80 kDa catalytic + ~28 kDa regulatory. Autoactivation cleaves the catalytic subunit N-terminus, producing a ~76 kDa fragment. See calpain-1 vs calpain-2 selection.
Gel and transfer conditions
Cathepsin two-chain forms have small light chains (5-8 kDa) that require a high-percentage gel (15% or 18%) or a gradient gel (4-20%) to retain. If you only run a 10% gel, you will not see the light chains — they run off with the dye front. For quantitative analysis of two-chain vs single-chain distribution, always use a gradient gel or a 15% resolving gel with a 4% stacking gel.
Calpains at ~80 kDa run well on 10% gels. Autoactivation cleavage of ~4 kDa is difficult to resolve on 10% but visible on 12% gels with careful loading.
Transfer: standard wet transfer at 100V for 60 minutes covers all cysteine proteinase MW ranges. For cathepsin light chains (5-8 kDa), reduce transfer time to 30 minutes at 100V or use a semi-dry transfer at 15V for 30 minutes — overlong transfer blows small proteins through the membrane.
Blocking and antibody incubation
Standard 5% non-fat dry milk in TBS-T (0.1% Tween-20) as default. Triple Point cysteine proteinase antibodies work well in either milk or BSA blocking — no phosphorylation issues to work around.
Working dilutions per CoA. Most cathepsin antibodies are supplied for use at 1:1,000 to 1:5,000. Overnight primary incubation at 4°C is standard.
Secondary: anti-rabbit HRP at 1:10,000, 1 hour room temperature. Standard ECL detection is usually sufficient because cathepsins are abundant in lysosome-containing cells. Escalate to enhanced ECL only if working with low-expression cell lines or primary neurons.
Detection strategies for specific questions
"Is the cathepsin activated?"
Use domain-specific antibodies. The propeptide-directed antibody detects only the procathepsin (~40-55 kDa depending on family member). The catalytic-domain-directed antibody detects both pro and mature. Loss of propeptide signal with retention of a lower-MW catalytic-domain band demonstrates activation. This is standard for the SPA-format kits — the RP1 antibody targets propeptide, RP2 targets catalytic domain.
"Is the cathepsin single-chain or two-chain?"
Use a heavy-chain-directed antibody and a light-chain-directed antibody in parallel. Single-chain mature shows a single ~30 kDa band on both. Two-chain shows the heavy-chain band on the heavy-chain antibody, the light-chain band (~5-8 kDa) on the light-chain antibody, and no band at the single-chain MW. The heavy:light ratio is diagnostic of late vs early lysosomal maturation.
"Is calpain activated?"
Autoactivation converts the ~80 kDa full-length catalytic subunit to a ~76 kDa fragment (N-terminal 4 kDa cleaved). This shift is visible on a 12% gel with good resolution. Alternative: use an autoactivation-site-specific antibody (some vendors sell them) that only detects the cleaved form. Loss of full-length signal + appearance of 76 kDa band is unambiguous activation. Note that calpain-1 and calpain-2 activation are similar in mechanism but not identical — see the calpain selection guide.
"Is the cathepsin secreted?"
Some cathepsins are secreted (particularly by cancer cells and osteoclasts) rather than retained in lysosomes. Probe both cell lysate and conditioned media. Secreted procathepsin (~40-55 kDa) appears in media; lysosomal mature forms remain intracellular. Increased media procathepsin with retained intracellular mature forms indicates active secretion, not simple lysosomal leakage.
Superpooled kits for cysteine proteinases
Triple Point Superpooled kits are available for the major cathepsins (B, C, D, K, L, S) and for calpain-1 and calpain-2. Each kit contains propeptide, catalytic-domain, and heavy/light-chain-directed polyclonals as separate reagents. See the SPA method overview and usage protocol for detail on running the pooled protocol.
Common failure modes specific to cysteine proteinases
Loss of full-length band during sample prep
Cathepsins autoactivated during lysis. Add E-64 to lysis buffer, use ice-cold buffer, boil in sample buffer immediately after lysis. This is the single most common cysteine proteinase failure mode.
Calpain-1 detection lost with high-calcium tissue
Calpain-1 autoactivated during tissue homogenisation, then autoproteolysed to fragments too small to detect. Homogenise tissue in EDTA-containing buffer at 4°C; consider adding calpain inhibitor peptides (calpastatin, MDL-28170) to the lysis buffer.
Light chain not visible on gel
Ran off with dye front. Use 15% or gradient gel; reduce transfer time to prevent blow-through; probe with a light-chain-specific antibody, not a heavy-chain-specific one.
Cross-detection between cathepsin B and cathepsin L
The two enzymes share ~55% catalytic-domain identity and antibodies raised against unselected catalytic-domain peptides commonly cross-detect. Triple Point's cathepsin B and cathepsin L antibodies use divergent peptide immunogens; cross-reactivity is measured and reported on the CoA. If your antibody is from another supplier and shows unexpected signal, run the paralogue-panel control described in the cathepsin B vs cathepsin D selection guide.
Bicarbonate-driven autoactivation
Cell culture media contains sodium bicarbonate; the acidic vesicles are stabilised by this buffer. Rinse cells thoroughly with PBS before lysis to remove media — residual bicarbonate can drive premature autoactivation.
Positive control samples for cysteine proteinases
Cathepsin B and L
Any lysosome-abundant cell type. Macrophages (RAW264.7, THP-1 differentiated), MDA-MB-231 breast cancer cells, HepG2 hepatocytes. Cathepsin B and L are nearly ubiquitous — most cell lines give strong signal.
Cathepsin K
Osteoclast-derived samples (RAW264.7 differentiated with RANKL, or primary osteoclasts). Cathepsin K is expressed at very low levels outside osteoclasts, so tissue extract requires enrichment.
Cathepsin S
Immune cells (macrophages, dendritic cells). Cathepsin S is elevated in antigen-presenting cells for MHC class II processing.
Calpains
Nearly all mammalian cells express calpain-1 and calpain-2 at detectable levels. HEK293, HeLa, or C2C12 myoblasts all give strong signal. Skeletal muscle is a particularly rich source for calpain-3.