An IP immunoprecipitation guide from sample to mass spectrometry asks which proteins remain with an endogenous bait after antibody capture, Input and IgG contrast, and LC-MS/MS identification. The path is readable only after the antibody recovers the native bait and every arm uses one elution-to-digest recipe. The work fits cells or tissues when an IP-competent antibody exists and the claim is co-recovery, not binary direct binding (Hein et al. 2015). Prefer this path when Input and IgG travel with the bait IP. Reassess when the antibody is Western-only, a Western elution blocks digestion, or a longer wash is used as a substitute for IgG.
Researchers who already have IP or Co-IP material and a written interaction claim can also review the MtoZ Biolabs IP-Mass Spectrometry Service for project-specific sample-to-MS review.
What This Sample-to-MS Path Can Claim
This guide is not a copied lysis-wash-elution recipe. The claim stays the same from the first lysate tube to the search table: protein B is enriched with bait A versus matched IgG, after bait recovery was shown versus Input. IP is that bait-recovery claim. Co-IP or IP-MS is the partner-ranking claim after recovery. Identification depth in a Co-IP-style eluate is typically hundreds to about a thousand proteins, depending on the sample. Depth is not a stronger interaction.
Protein-DNA or protein-RNA capture, drug-target screens, and PTM mapping are different experiment classes. They are not completed by adding mass spectrometry to a generic IP cartoon. How matched arms stay comparable through digestion is covered in How to Prepare Immunoprecipitation Samples for Mass Spectrometry.
The path fits cultured cells or tissues when the antibody is supplied by the study and a protein reference database exists. It is a poor first path when tagging is required because no IP-competent antibody exists, or when the claim is reconstituted binding of a purified domain.
An IP path from sample to MS reports candidate co-recovery under stated controls. It does not certify that two proteins contact each other.
Antibody Specificity Before the First Wash
Antibody specificity is the first sample-to-MS gate. If the capturing reagent does not recover the intended native bait, later cleanup and a longer LC run will not create an IP-MS result. Application-specific validation remains necessary because Western performance does not establish IP performance (Uhlen et al. 2016). How to qualify that reagent is covered in How to Choose the Right Antibody for Co-IP Assays?.
Record clone, lot, host species, and bead pairing next to the sample set. Extra antibody that fills the eluate with chain signal makes both blot and IP-MS noisier. Empty-tag controls from AP-MS do not replace antibody documentation on this path. Do not raise lysate input only to compensate for a Western-only clone.
If no IP-competent antibody exists, stop the IP-to-MS path and change handle. Tagged AP-MS is a different analysis class.
Input and IgG Must Travel From Sample to Search
Input and IgG are the denominators of every step in this guide. They are not optional after the bait IP looks clean.
Input is taken before capture. It shows whether the bait existed in the extract and provides a baseline for enrichment. A missing bait protein group is an expression or extraction problem if Input is empty. It is a capture or elution problem if Input is positive.
IgG or bead controls must use the same lysate batch, lysis, wash, elution, digestion, and search. Preclearing is not a substitute for IgG. Increasing wash count on the bait IP only, or eluting IgG unlike the bait IP, makes enrichment unreadable. PBS or TBST names do not replace that matched recipe.
Sample-to-MS check |
What a pass looks like |
What the path cannot say |
|---|---|---|
Antibody specificity |
Intended bait recovered; no dominant off-target |
Partner names from an off-target capture |
Input |
Bait present before capture when expected |
A missing IP read as a missing interaction |
IgG or bead |
Same wash, elution, and digest as the bait IP |
Every remaining name is bait-specific |
Enrichment efficiency |
Bait higher in bait IP than in Input and IgG |
A longer wash as a finished interactome |
MS compatibility |
Eluate digestible after the planned cleanup |
Residual SDS or salt left in the digest |
A practical path is:
- Write the co-recovery sentence the MS table must support.
- Confirm native bait recovery versus Input on a pilot elution.
- Apply that same elution and digest to matched IgG.
- Rank only names that pass that contrast.
- Keep a shortlist for an orthogonal pair assay.
Figure 1. An IP guide from sample to MS starts after bait recovery is shown. A wash cartoon is not an interpreted result.
Enrichment Efficiency and Mass Spectrometry Credibility
Enrichment efficiency decides whether capture worked before MS is interpreted. If the bait is weak versus Input, stop partner ranking. Relative intensity can compare the same protein across Input, IgG, and bait IP only when every arm used one recipe. Intensity does not rank unrelated proteins as tighter binders.
Acid elution, denaturing elution plus cleanup, or on-bead digestion can all finish the path when they leave a digestible, bait-positive sample. SDS-PAGE loading buffer does not go straight to LC-MS/MS. Residual SDS above a digestible level blocks trypsin. Protocol-level search matching is covered in Immunoprecipitation Mass Spectrometry Protocol.
For IP-MS, identification of a protein is not a confidence call. Ranking requires bait recovery, Input-aware enrichment, and a declared contrast against IgG. Probabilistic scoring such as SAINT uses bait versus control counts to assign a probability that a prey is specifically co-purified (Choi et al. 2011). A contaminant catalog such as the CRAPome is a prior on control frequency (Mellacheruvu et al. 2013). Pathway software can annotate a shortlist. It cannot repair a missing IgG arm or convert a generic IP cartoon into a validated interactome.
An IP path from sample to MS supports candidate co-recovery after antibody, Input, IgG, and bait enrichment are shown. It does not certify a binary interactome.
Figure 2. Mass spectrometry credibility is a path outcome. Extra washing does not replace Input and IgG.
Common Guide Biases and Result Boundaries
The common bias is writing IP as highly specific because an antibody was used, then treating more washes as the solution to every background band. Another bias is sending only the bait IP to MS and reading leftover names as an interaction network. Over-washing can erase the association the claim needed. Under-washing, or a Western elution left in the tube, makes bait IP and IgG similar or undigestible.
Do not write a successful bait IP as proof that remaining proteins contact the bait. Do not write a negative IP-MS table as proof that no interaction exists in the cell. Absence can mean the wrong antibody, epitope masking, over-washing, or a bait that is not expressed in the tested state.
Prefer this sample-to-MS path when native bait recovery can be shown and every arm matches. Prefer tagged AP-MS when no IP-competent antibody exists. Prefer a defined-pair assay when the next sentence is direct contact.
Figure 3. Keep the IP immunoprecipitation path only while native bait recovery is documented. A pathway map is not a completed guide.
Once the claim, antibody status, matched Input and IgG, elution chemistry, and the planned digest are defined, the project can be scoped as IP or IP-MS from sample to mass spectrometry. Researchers can review the MtoZ Biolabs IP-Mass Spectrometry Service for sample evaluation and path planning.
Frequently Asked Questions
1. Is this guide a fixed wash and elution recipe?
No. Choose chemistry that recovers the bait, matches IgG, and can be digested. A copied PBS or SDS loading buffer is not a universal answer.
2. Can I send only the bait IP from sample to MS?
No. Input and IgG must use the same elution and digest. One eluate tube is not an IP-MS result.
3. Does more washing replace an IgG control?
No. Extra washes on the bait IP only make enrichment unreadable. IgG must see the same recipe.
4. Can SDS-PAGE sample buffer go straight to LC-MS/MS?
Not as a leftover solution. Gel-based digestion or SDS removal is required first.
5. When should this path move to a pair assay?
Move when the next written sentence is direct contact or affinity. Keep the MS shortlist as the input to that assay, not as the proof.
Reference
- M. Uhlen, A. Bandrowski, S. Carr, A. Edwards, J. Ellenberg, E. Lundberg, D. L. Rimm, H. Rodriguez, T. Hiltke, M. Snyder, T. Yamamoto (2016). A proposal for validation of antibodies. Nat. Methods, 13, 823-827. https://doi.org/10.1038/nmeth.3995
- M.Y. Hein, N.C. Hubner, I. Poser, J. Cox, N. Nagaraj, Y. Toyoda, I.A. Gak, I. Weisswange, J. Mansfeld, F. Buchholz, et al. (2015). A human interactome in three quantitative dimensions organized by stoichiometries and abundances. Cell, 163, 712-723. https://doi.org/10.1016/j.cell.2015.09.053
- H. Choi, B. Larsen, Z.Y. Lin, A. Breitkreutz, D. Mellacheruvu, D. Fermin, Z.S. Qin, M. Tyers, A.C. Gingras, A.I. Nesvizhskii (2011). SAINT: probabilistic scoring of affinity purification-mass spectrometry data. Nat. Methods, 8, 70-73. https://doi.org/10.1038/nmeth.1541
- D. Mellacheruvu, Z. Wright, A.L. Couzens, J.P. Lambert, N.A. St-Denis, T. Li, Y.V. Miteva, S. Hauri, M.E. Sardiu, T.Y. Low, et al. (2013). The CRAPome: a contaminant repository for affinity purification-mass spectrometry data. Nat. Methods, 10, 730-736. https://doi.org/10.1038/nmeth.2557
