Protein Recognition and Assembly by a Phosphocavitand

Controlled protein assembly is an enabling technology, in particular, for biomaterials fabrication. Here, we report protein recognition and assembly by a phosphate-containing macrocycle (pctx). We show that the C₃-symmetric phosphocavitand is a versatile receptor for N-terminal residues or arginine but not lysine. Using atomic resolution X-ray diffraction data, we reveal the precise details of N-terminal complexation in the β-propeller protein Ralstonia solanacearum lectin (RSL). In some cocrystal structures, a tetrahedral cluster of the phosphocavitand occupies one end of the β-propeller fold, providing a node for protein assembly. The macrocycle cluster is compatible with different types of precipitants, a broad pH range, and zinc complexation. We demonstrate system control with an arginine-enriched RSL that alters the overall assembly due to selective arginine complexation by pctx. A lysozyme-pctx cocrystal structure also demonstrates arginine complexation by the macrocycle. An alternative macrocycle cluster occurs with an engineered RSL bearing an extended N-terminus. In this structure, involving zinc ligation at the N-terminus, the macrocycle forms trimeric clusters and four such clusters form cage-like substructures within the tetrahedral protein framework. Thus, N-terminal complexation in combination with phosphocavitand self-assembly provides new routes to protein crystal engineering.