Proximity labeling mass spectrometry (PL-MS) names proteins that were covalently biotinylated near a fused enzyme in a living cell and then recovered on streptavidin. The enzyme is a biotin ligase (BioID, TurboID, miniTurbo) or a peroxidase (APEX2). Liquid chromatography–tandem MS (LC-MS/MS) identifies that streptavidin eluate. The table is a neighborhood or a compartment proteome. It is not a protein–protein interaction, a natural spatial distribution, or a dynamic cellular map. Proximity is not binding. Labeling efficiency does not guarantee specificity. An enzyme-only arm, matched for localization, is part of the method. Without it, neighbors of an overexpressed ligase or peroxidase are scored as interactors.
Cancer-network, synapse, immune-pathway, diagnostic-marker, vaccine, and environmental-science sentences are later overlays. They are not applications granted by biotinylation. Yeast two-hybrid and co-immunoprecipitation (Co-IP) ask different questions. In-cell labeling does not make PL-MS a more accurate report of those questions.
Researchers who already have a BioID, TurboID, or APEX2 fusion and need a streptavidin table can also review the MtoZ Biolabs Proximity Labeling Mass Spectrometry Service for project-specific feasibility and analysis planning.
Common Proximity Labeling Methods
Two enzyme classes share streptavidin–MS and do not share a pulse.
Biotin ligase–mediated labeling (BioID, TurboID)
The bait is fused to a promiscuous biotin ligase, historically BirA* in BioID and the faster TurboID or miniTurbo variants. Added biotin is attached to nearby lysines. BioID usually needs many hours. TurboID can label in about ten minutes. The often-quoted radius near 10 nm is a working estimate. Higher ligase activity shortens the window and often raises enzyme-only biotinylation. For how TurboID is used when Co-IP occupancy is too low, How Can TurboID-Based Proximity Labeling Enable the Study of Low-Abundance Protein Interactions? provides a complementary TurboID page.
Peroxidase-mediated labeling (APEX2)
The bait is fused to APEX2. Biotin-phenol plus hydrogen peroxide (H₂O₂) produces a short-lived radical that marks nearby tyrosines, typically in under a minute. A working radius near 20 nm is often quoted. H₂O₂ is an oxidant pulse. For that chemistry, APEX Proximity Labeling provides a complementary APEX2 page. For how the streptavidin file is read as a proteome, APEX Proteomics provides a complementary proteomic reading.
Nucleic-acid–tethered labeling exists in some designs. This page is enzyme-fusion PL-MS. Antibody–enzyme fusions are a different construct and still need an enzyme-only or isotype-enzyme arm.
Figure 1. BioID/TurboID is a ligase pulse. APEX2 is a peroxidase pulse. Both yield a streptavidin table. Enzyme-only estimates background in either class.
Choose the enzyme class before the first transfection. A one-minute APEX2 pulse and an overnight BioID pulse are not interchangeable mid-study.
Proximity Labeling Mass Spectrometry Experimental Process
These five steps are the shared wet order. The enzyme-only arm travels through every step.
1. Construct the fusion and the enzyme-only vector
The bait gene is joined to BirA*, TurboID, or APEX2. A localization-matched enzyme without the cargo, or a catalytically inactive mutant, is built at the same time. Adding that arm after a single bait digest cannot reconstruct background.
2. Express at a checked localization
The fusion is introduced by transfection or a stable line. Overexpression and aggregation biotinylate by crowding. Localization is checked on the same cells before the pulse.
3. Run the labeling reaction
Biotin is added for a ligase. Biotin-phenol and H₂O₂ are added for APEX2. A no-substrate arm estimates probe background. Time and dose are titrated. They are not a recipe that ensures accurate labeling of target molecules.
4. Enrich on streptavidin
Streptavidin beads recover biotinylated proteins. They do not purify away all contaminants. Endogenous biotinylated carboxylases and bead binders remain. On-bead digestion or a denaturing release is the usual exit. Mild elution does not reverse streptavidin–biotin.
5. Analyze by LC-MS/MS, then enrich against enzyme-only
Fragment spectra name proteins in the digest. Parent mass is not an identification. Each row is first asked whether the enzyme or bait was recovered and whether the candidate is enriched against enzyme-only. Subtracting every control identification deletes real neighbors that leak into the enzyme-only arm. Annotation of cancer, synapse, or immune roles comes last.
Figure 2. Vector, expression, pulse, streptavidin, LC-MS/MS. The search engine cannot invent an enzyme-only digest that was never run.
What PL-MS Reports, and What It Does Not
PL-MS can support a live-cell neighborhood or a compartment proteome when the enzyme stays on site and enzyme-only is run. Low-abundance proteins appear if they were biotinylated and enriched. Sensitivity of the spectrometer does not convert presence into an interaction.
Co-IP still reports a wash-stable pellet and can be endogenous. Yeast two-hybrid still reports a reporter in yeast. Cross-linking MS still reports a residue pair. PL-MS is not a more accurate version of those three. For when a partner list versus a residue pair is the next sentence, Co-IP-MS vs XL-MS: Which Protein Interaction Workflow Should You Choose? provides a complementary method choice.
Figure 3. In-cell labeling is not a more natural interaction. Drug-target, vaccine, and diagnostic sentences are not products of streptavidin enrichment.
Experimental Limits
PL-MS reduces a living cell to what a fused enzyme, a substrate pulse, streptavidin, and the search engine can report. Diagnostic language sits outside ordinary research reporting unless a separately validated assay framework is in place.
Project Planning and Service Scope
A usable PL-MS plan names the enzyme class, the enzyme-only arm, the pulse, and the claim that will be allowed, before the first transfection. MtoZ Biolabs scopes BioID, TurboID, and APEX2 with Orbitrap-class LC-MS/MS. Interpretability still follows those arms, not a broader application list.
Once the next sentence is a wash-stable endogenous pellet, the project should not be forced through a ligase or peroxidase. Researchers who need antibody capture can review the MtoZ Biolabs Co-Immunoprecipitation Protein Interaction Analysis Service for project-specific sample evaluation, feasibility assessment, and workflow planning.
Frequently Asked Questions
1. What does proximity labeling mass spectrometry identify?
Proteins biotinylated near a fused ligase or peroxidase and recovered on streptavidin. Interaction language requires enzyme-only enrichment.
2. Is BioID the same as APEX?
No. BioID/TurboID mark lysines with biotin over minutes to hours. APEX2 marks tyrosines with biotin-phenol and H₂O₂ in about a minute.
3. Does in-cell labeling reflect natural interactions more accurately than Co-IP?
It records proximity before lysis. It does not prove a natural complex and it is not a substitute for an endogenous Co-IP.
4. Can PL-MS supply cancer or synaptic drug targets?
Not from one streptavidin table. Those claims need orthogonal assays.
5. How does this page differ from the APEX and TurboID pages?
This page is the shared family and the five-step order. The APEX and TurboID pages are the enzyme-specific chemistry and use-cases.
Reference
- Roux K.J., Kim D.I., Raida M., Burke B. (2012). A promiscuous biotin ligase fusion protein identifies proximal and interacting proteins in mammalian cells. J. Cell Biol., 196, 801-810. https://doi.org/10.1083/jcb.201112098
- Branon T.C., Bosch J.A., Sanchez A.D., et al. (2018). Efficient proximity labeling in living cells and organisms with TurboID. Nat. Biotechnol., 36, 880-887. https://doi.org/10.1038/nbt.4201
- Rhee H.W., Zou P., Udeshi N.D., et al. (2013). Proteomic mapping of mitochondria in living cells via spatially restricted enzymatic tagging. Science, 339, 1328-1331. https://doi.org/10.1126/science.1230593
- Hung V., Udeshi N.D., Lam S.S., et al. (2016). Spatially resolved proteomic mapping in living cells with the engineered peroxidase APEX2. Nat. Protoc., 11, 456-475. https://doi.org/10.1038/nprot.2016.018
- Gingras A.C., Abe K.T., Raught B. (2019). Getting to know the neighborhood: using proximity-dependent biotinylation to characterize protein complexes and map organelles. Curr. Opin. Chem. Biol., 48, 44-54. https://doi.org/10.1016/j.cbpa.2018.10.017
