Crystallization of VHL-based PROTAC-induced ternary complexes.

X-ray crystal structures of PROTAC-induced ternary complexes provide invaluable insights into the critical species underpinning PROTAC mode of action, explain protein degradation selectivity profiles, and can guide rational degrader design. Nevertheless, crystallization of the ternary complexes formed by PROTACs remains an important bottleneck in employing this method. This is mainly due to the potential flexibility and heterogeneity that is inherent to a non-native protein-protein complex mediated by a small molecule, which together can hamper crystallization of the desired species. To overcome this limitation, selecting PROTAC compounds that enable the formation of stable, high-affinity and preferably cooperative ternary complexes in stoichiometric amount is, in our experience, critical to the success of co-crystallization studies. In this chapter, examples of stable PROTAC-mediated ternary complexes are illustrated. Learnings from biophysical & biochemical data are used as a guideline in achieving the highest "crystallizability" of ternary complexes. A case study of VHL-based SMARCA2 PROTAC degrader ternary complex crystallization is described. The procedure includes over-expression and purification of the E3 ligase and target protein, forming (and sometimes isolating) the ternary complex, and crystallizing it. The protocols can be applied for other combinations of E3 ligase, PROTAC and target protein.