How Tau and Fyn lock your PSD condensate in place

Proteins inside neurons do not just float freely. At every synapse, thousands of them pack into a dense region called the postsynaptic density, or PSD. Think of the PSD as a tiny circuit board: it holds receptors in place, routes incoming signals, and adjusts the strength of the synapse over time. The proteins in that board need to stay somewhat mobile so the synapse can adapt. When that mobility goes, signaling falters.

A new preprint shows, in precise molecular detail, how two proteins, Tau and Fyn, team up to shut down that mobility inside PSD condensates. Both proteins are already linked to synaptic dysfunction in Alzheimer's disease and related conditions. Now there is a structural explanation for how they do it.

Building a PSD outside the cell

To study the PSD in a controlled setting, the team reconstituted it from purified proteins in a test tube. The resulting structures are called biomolecular condensates. These are small protein-rich droplets that form through phase separation, the same basic idea as oil separating from water in a salad dressing. The condensate interior is dense and gel-like; the surrounding solution is dilute.

Inside these artificial condensates, both Tau and Fyn concentrated at higher levels than the core PSD scaffold proteins, including PSD-95, the central organizer of the postsynaptic density. That enrichment tells you these two proteins actively insert themselves into the hub. They are not spectators sitting at the edges.

FRAP: measuring the freeze

The researchers used FRAP (fluorescence recovery after photobleaching) to measure how freely proteins moved inside the condensates. The method works like this: tag your protein with a glowing fluorescent label, then use a laser to permanently bleach a small spot. If proteins can move, fresh glowing copies drift in and the signal recovers. Slow or absent recovery means the proteins are stuck.

Tau alone reduced Fyn's mobility. Tau and Fyn together did something stronger: they cooperatively arrested the movement of PSD-95 itself, the scaffold that holds the whole assembly together. The arrest required both proteins at once. Neither achieved the same effect alone.

The key residues are proline 216 and proline 219 on Tau (written as the P216A/P219A mutation, meaning those prolines are swapped for alanines). That single pair of changes nearly completely restored the mobility of both Fyn and PSD-95. Remove the hotspot, lose the arrest.

A 1.4 ångström picture of the interaction

NMR spectroscopy (a technique that maps molecular structure by measuring how atomic nuclei respond to magnetic fields) showed that Tau's proline-rich region makes multiple contacts with a small module on Fyn called the SH3 domain. SH3 domains specialize in binding proline-rich sequences, and they appear across dozens of signaling proteins. Tau carries several of those binding motifs along its proline-rich stretch, so one Tau molecule can engage the Fyn SH3 domain at more than one site simultaneously.

The team solved a crystal structure of the Fyn SH3 domain bound to a Tau peptide at 1.4 ångströms resolution, close to atomic-level detail. The structure shows a 2:1 arrangement: two Fyn SH3 domains gripping one Tau molecule at the same time. When Tau bridges two Fyn molecules inside a condensate, and those Fyn molecules are also interacting with the condensate network around them, you get a crosslinked mesh. Many moderate contacts working together produce a stickiness that no single contact could. That is what FRAP records as dynamic arrest.

One more point worth flagging: the kinase activity of Fyn, its ability to transfer phosphate groups to other proteins, is not required for condensate partitioning. An SH3-SH2 fragment of Fyn without the kinase domain enriched inside the condensates just as well. The locking effect comes from binding geometry, not enzymatic function.

Bench implications for Tau peptide work

If your research uses Tau-derived peptides in binding or reconstitution assays, the P216/P219 region is the contact zone that drives the behavior. A Tau fragment spanning those two prolines engages the Fyn SH3 domain through a multivalent mechanism. A fragment that truncates or mutates them will behave very differently, and comparing results across fragment lengths without accounting for this will produce confusing data.

• Verify purity before condensate assays. Multivalent interactions are cooperative: truncated or misfolded peptide contaminants can break the crosslinking geometry and produce falsely weak binding signals. Mass spectrometry confirmation before your assay catches this early.
• Minimize freeze-thaw cycles on Tau stocks. Proline-rich regions are relatively stable, but oxidation and deamidation accumulate at room temperature. Store at or below -20 °C and use single-use aliquots reconstituted in bacteriostatic water or the appropriate assay buffer.
• Match your reconstitution buffer carefully. Phase separation boundaries shift with salt concentration and pH. Small buffer mismatches change whether your condensate forms at all, and that affects every mobility measurement downstream.
• Check for aggregation before FRAP runs. Aggregated Tau peptide can arrest condensate dynamics independently of any Fyn interaction. A dynamic light scattering check or a brief pellet spin before the assay will catch pre-formed aggregates before they corrupt your mobility data.

The 1.4 ångström structure gives the peptide research community a precise target. If you want to probe or disrupt the Tau-Fyn interaction in your condensate model, you now know exactly which residues to focus on, and which mutations serve as a near-complete negative control.

Frequently asked questions

What is dynamic arrest in a PSD condensate?

Scaffold proteins like PSD-95 lose their ability to move freely inside the condensate droplet. The normally fluid assembly becomes nearly immobile, which disrupts how the postsynaptic density processes and routes signals.

Why are prolines 216 and 219 on Tau so important?

They are the primary binding hotspot between Tau and the Fyn SH3 domain. Mutating both to alanine (P216A/P219A) nearly completely restores protein mobility in condensates, confirming they drive the cooperative arrest.

Does Fyn kinase activity cause it to partition into PSD condensates?

No. The SH3-SH2 fragment of Fyn without the kinase domain enriches inside condensates just as well. Partitioning and dynamic arrest are driven by the SH3-Tau proline-rich region interaction, not enzymatic function.

Prompted by this coverage at bioRxiv →

Related from our lab: the pen · cartridges · bacteriostatic water · reconstitution calculators

Shared by PreppinPeppers for research, educational, and demonstration awareness only. We link to third-party coverage; we do not endorse it, and nothing here is medical advice or a recommendation to use any substance in humans or animals. Our products are sold for laboratory research use only.

What the research community gets wrong about the Tau-Fyn interaction

• It is not one tight lock. The Tau proline-rich region and the Fyn SH3 domain hold together through many moderate contacts working at once (a multivalent interaction), not a single high-affinity grip. That is why swapping just two prolines (P216A/P219A) can undo most of the binding in a condensate assay. Small edits to your peptide fragment matter more than people expect.
• Kinase activity is not the same as binding. Fyn's enzyme (kinase) function is not needed for it to concentrate inside the condensate. An SH3-SH2 fragment with no kinase domain enriches just as well, so a kinase-dead Fyn is not a binding-negative control. If you want a real negative control, mutate the proline hotspot on Tau instead.
• The proline-rich region is not floppy filler. In binding and reconstitution work, those prolines are the actual contact zone (the same region reported to engage Src-family SH3 domains). Truncating or mutating them changes the result, so treat that stretch as functional, not spacer.
• Different fragment lengths are not interchangeable. Because the binding is multivalent, a shorter Tau fragment can drop contact sites and read as weak binding even when the full proline-rich region binds well. Comparing results across fragment lengths without noting this produces confusing data.
• Slow FRAP recovery is not always a Fyn effect. Pre-formed Tau aggregate in your stock can arrest condensate movement on its own. Blaming slow recovery on the Tau-Fyn interaction, when it is really aggregate, is a common mix-up. A quick aggregate check before the run saves you here.

From our bench: If you run FRAP on a reconstituted PSD condensate, we want to compare recovery half-times for a wild-type Tau fragment versus a P216A/P219A fragment prepared from the same stock, buffer, salt, and pH. Send us your paired half-times (with the buffer conditions and how you confirmed the fragments were aggregate-free), and we will log the observation so other researchers can see how much of the mobility arrest tracks with the proline hotspot at the vial-and-condensate level.

Sources

1. Bacteriostatic Water for Injection, USP , FDA/DailyMed label (0.9% benzyl alcohol)
2. Duerkop et al., Biotechnol J 2018 , Impact of Cavitation, High Shear Stress and Air/Liquid Interfaces on Protein Aggregation
3. Sigma-Aldrich (Merck) , Handling and Storage Guidelines for Peptides and Proteins
4. UniProt P10636 , Microtubule-associated protein tau (MAPT), Homo sapiens
5. UniProt P06241 , Tyrosine-protein kinase Fyn (FYN), SH3 domain (aa 82-143), Homo sapiens
6. Lee G, et al. Tau interacts with src-family non-receptor tyrosine kinases. J Cell Sci. 1998;111(21):3167-77 (PMID 9763511)