How to Build a PROTAC

A PROTAC usually contains three modular parts: a ligand for the protein of interest, a linker, and an E3 ligase recruiter. The degrader works only if those parts support a productive ternary complex rather than two disconnected binary interactions.

• The POI ligand binds the target protein.
• The E3 recruiter binds an endogenous E3 ligase.
• The ternary complex enables E2 or E3-mediated ubiquitin transfer to the POI.
• Polyubiquitinated POI is sent to the proteasome for degradation.
• The PROTAC may then dissociate and engage another target molecule.

A good PROTAC project starts with target selection, not linker shopping. The POI should have a strong biological rationale and a reason why degradation may be more informative or more therapeutic than inhibition alone.

• Degradation can remove scaffolding or non-enzymatic functions that inhibitors do not fully address.
• Mutant-selective or disease-selective contexts may matter for safety, selectivity, or therapeutic rationale.
• The best binder does not always need to hit an active site if it can support productive ternary geometry.
• Cellular localization, expression level, turnover rate, lysine accessibility, and assay availability all influence feasibility.

The E3 recruiter determines which ligase you bring into the ternary complex and therefore influences geometry, cooperativity, degradation profile, and tissue or cell-context suitability.

• VHL and CRBN are common starting points, but not universal answers.
• E3 choice should be considered alongside cell type, tissue expression, biology, and known off-target liabilities.
• Recruiter geometry can strongly reshape which ternary complexes are accessible.

If recruiter-side uncertainty is high, inspect recruiter scaffold context and ligase-side attachment options before you spend too much effort on linker enumeration.

Do not assume one linker is best. Start with a small panel that varies length, flexibility, polarity, rigidity, and attachment chemistry so you can learn how the POI and recruiter tolerate different bridge geometries.

Acceleration often comes from scale. DNA-encoded libraries, connector libraries, and batch workflows can help probe many POI or recruiter combinations much faster than serial one-off chemistry.

• Binary binding screens tell you whether one side of the molecule still engages.
• Ternary complex screens ask a different, often more relevant, question.
• Batch or API workflows help standardize larger candidate enumerations before synthesis or testing.

Computational prioritization can help, especially when you have too many candidates to test immediately. Useful downstream methods include docking, restrained ternary modeling, molecular dynamics, and structure-guided refinement, but they should rank hypotheses rather than declare biological truth.

PROTACs are evaluated by degradation behavior, not only by occupancy. A useful degrader needs enough cellular engagement to produce degradation, enough maximal effect to matter biologically, and enough selectivity to support interpretation.

• DC50: concentration associated with half-maximal degradation.
• Dmax: maximum observed degradation.
• Hook effect: reduced degradation at high PROTAC concentrations when excess binary complexes compete with productive ternary complex formation.

At a high level, degradation can be followed by Western blot, targeted proteomics, reporter systems, luminescence-style readouts, or broader cellular degradation assays depending on the project.

Direct-to-biology is best thought of as a discovery accelerator. It can reveal exit-vector trends, linker trends, and promising hit PROTACs faster by coupling parallel chemistry with rapid biological readouts.

• It is useful for trend detection, not final proof.
• Assays must be robust enough to tolerate mixtures and impurity-related noise.
• Purified compound confirmation is still required before strong conclusions are drawn.