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Short answer
Bimolecular fluorescence complementation (BiFC) tests whether two proteins interact inside living cells. A fluorescent protein is split into two non-fluorescent fragments, each fused to one protein of interest. If the two proteins bind, the fragments are brought together, the fluorescent protein reconstitutes, and the cell lights up. A green signal is therefore evidence of proximity, and only well-controlled experiments make it evidence of interaction.
Definition and scope
BiFC is built on one principle: fluorescence recovery requires the interaction of two proteins. Neither fragment fluoresces alone. Each is fused to a different candidate partner, and the pair is expressed in the same cell. When the partners associate, the N-terminal and C-terminal fragments approach each other and recombine into a fluorescent protein (Miller et al.; Ren et al. 2024).
It is a live-cell, imaging-based method. Miller et al. list low background, no need for fixation, and compatibility with a standard epifluorescence microscope as practical advantages, and note that the approach suits large screens. Because the reassembled complex is stable once formed, BiFC can also capture weak or transient interactions that other approaches miss.
How the split fragments report an interaction
Fragment choices that have been used
The split point matters, because it decides whether the fragments stay dark on their own and still reassemble when pulled together. Miller et al. record these examples:
| Fluorescent protein | Reported split |
|---|---|
| GFP | Split in a loop between residues 157 and 158 |
| YFP | Split between amino acids 154 and 155, which gave the highest fluorescence in the cited work |
| Venus | Split at D173 in some systems |
| mCherry | Fragments named MN159 and MC160 |
Different colours of fragment pairs allow multicolour BiFC, which supports monitoring several interactions at once and comparing their interaction efficiencies in the same cells (Ren et al. 2024).
Why the signal is stable
Complex formation is largely irreversible. That is the source of the method’s sensitivity to weak interactions, and also of its main weakness, covered below.
A controls checklist
Miller et al. and Ren et al. describe the following controls, which help separate a real interaction signal from fragment artifacts.
- Interface-mutant negative control. Mutate the binding partner at the interface so it should no longer interact, and confirm the signal drops. Ren et al. likewise list a non-interacting mutated partner as a negative control.
- Single-fusion and fragment-only cells. Express each fragment, or each fusion paired with an empty fragment, to show that no signal appears without the second partner (Ren et al. 2024).
- Positive control pair. Ren et al. describe a bJun and bFos co-transfection (pBiFC-bJunVN173 with pBiFC-bFosVC155) as a positive control.
- Competition with untagged partner. Where no structure is known to guide a mutation, overexpress an untagged version of the partner and check that BiFC signal falls (Miller et al.).
- Orthogonal validation. Confirm the interaction with an independent assay; Miller et al. recommend this because false positives can still occur even with controls.
What a signal does and does not prove
Irreversible assembly
Because the fragments do not come apart, BiFC does not report dissociation, which limits its use for monitoring dynamics. Ren et al. note that irreversible complementation can produce artifacts when interactions are weak and transient.
Self-assembly of fragments
Fragments that assemble without any help from the fused proteins give false positives. Ren et al. report that mutations near the C-terminus of sfGFP reduce self-assembly, as does splitting monomeric Venus (mVenus) at residue 210.
Overexpression
In yeast, expression from native promoters appeared to give a reversible complex, which Miller et al. take to suggest that overexpression may contribute to the irreversibility seen elsewhere.
Maturation and location
Per Miller et al., slow chromophore maturation can hide transient interactions, and the fluorescent signal may not mark the true site of the interaction. Some EYFP fragments need low-temperature pre-incubation before the signal appears.
Where it matters for labs
BiFC needs only a standard epifluorescence microscope and the plasmids for the fusions, and Miller et al. describe it as suited to large screens. It sits alongside other interaction methods that CASRAI covers: the yeast two-hybrid assay, co-immunoprecipitation, proximity ligation assay and FRET microscopy. Ren et al. describe co-immunoprecipitation as constrained in what it can show about the transient and dynamic nature of interactions. For background on the labels themselves, see the guide to fluorophores.
Common misunderstandings
- “Fluorescence means the proteins bind each other directly.” The signal shows the fragments came together; it does not rule out self-assembly or overexpression artifacts.
- “BiFC shows when an interaction ends.” Largely irreversible assembly means it does not.
- “The brightest signal marks the interaction site.” The signal may not mark the true site of interaction.
- “One negative control is enough.” Miller et al. and Ren et al. describe several controls, and Miller et al. add independent validation.
FAQ
What does BiFC stand for?
Bimolecular fluorescence complementation. The name refers to two molecules, each carrying half of a fluorescent protein, that complement each other when they meet.
Does BiFC work in living cells?
Yes. It is described as suited to live-cell imaging without fixation.
Can BiFC detect weak or transient interactions?
It can, because the complex is stable once formed. The same stability means it cannot easily report that an interaction has ended.
What negative controls are used in BiFC?
The reviews describe a binding partner mutated at the interface, cells expressing only fragments, and cells expressing one fusion alone.
Sources
- Miller KE, Kim Y, Huh WK, Park HO. Bimolecular fluorescence complementation (BiFC) analysis: advances and recent applications for genome-wide interaction studies. https://pmc.ncbi.nlm.nih.gov/articles/4417415/ (checked 2026-10-29).
- Ren et al. 2024. Comprehensive review on BiFC in cell signaling pathways. Biomolecules. https://pmc.ncbi.nlm.nih.gov/articles/PMC11274695/ (checked 2026-10-29).
