Toward the computational design of protein crystals with improved resolution

Accurate computational design of three-dimensional protein crystals

Protein crystallization plays a central role in structural biology. Despite this, the process of crystallization remains poorly understood and highly empirical, with crystal contacts, lattice packing arrangements and space group preferences being largely unpredictable. Programming protein crystallization through precisely engineered side-chain-side-chain interactions across protein-protein interfaces is an outstanding challenge. Here we develop a general computational approach for designing three-dimensional protein crystals with prespecified lattice architectures at atomic accuracy that hierarchically constrains the overall number of degrees of freedom of the system. We design three pairs of oligomers that can be individually purified, and upon mixing, spontaneously self-assemble into >100 µm three-dimensional crystals. The structures of these crystals are nearly identical to the computational design models, closely corresponding in both overall architecture and the specific protein-protein interactions. The dimensions of the crystal unit cell can be systematically redesigned while retaining the space group symmetry and overall architecture, and the crystals are extremely porous and highly stable. Our approach enables the computational design of protein crystals with high accuracy, and the designed protein crystals, which have both structural and assembly information encoded in their primary sequences, provide a powerful platform for biological materials engineering.

Computational engineering of previously crystallized pyruvate formate-lyase activating enzyme reveals insights into SAM binding and reductive cleavage

Radical S-adenosyl-l-methionine (SAM) enzymes are ubiquitous in nature and carry out a broad variety of difficult chemical transformations initiated by hydrogen atom abstraction. Although numerous radical SAM (RS) enzymes have been structurally characterized, many prove recalcitrant to crystallization needed for atomic-level structure determination using X-ray crystallography, and even those that have been crystallized for an initial study can be difficult to recrystallize for further structural work. We present here a method for computationally engineering previously observed crystallographic contacts and employ it to obtain more reproducible crystallization of the RS enzyme pyruvate formate-lyase activating enzyme (PFL-AE). We show that the computationally engineered variant binds a typical RS [4Fe-4S]2+/+ cluster that binds SAM, with electron paramagnetic resonance properties indistinguishable from the native PFL-AE. The variant also retains the typical PFL-AE catalytic activity, as evidenced by the characteristic glycyl radical electron paramagnetic resonance signal observed upon incubation of the PFL-AE variant with reducing agent, SAM, and PFL. The PFL-AE variant was also crystallized in the [4Fe-4S]2+ state with SAM bound, providing a new high-resolution structure of the SAM complex in the absence of substrate. Finally, by incubating such a crystal in a solution of sodium dithionite, the reductive cleavage of SAM is triggered, providing us with a structure in which the SAM cleavage products 5'-deoxyadenosine and methionine are bound in the active site. We propose that the methods described herein may be useful in the structural characterization of other difficult-to-resolve proteins.

Redefining Protein Interfaces within Protein Single Crystals with DNA

Proteins are exquisite nanoscale building blocks: molecularly pure, chemically addressable, and inherently selective for their evolved function. The organization of proteins into single crystals with high positional, orientational, and translational order results in materials where the location of every atom can be known. However, controlling the organization of proteins is challenging due to the myriad interactions that define protein interfaces within native single crystals. Recently, we discovered that introducing a single DNA-DNA interaction between protein surfaces leads to changes in the packing of proteins within single crystals and the protein-protein interactions (PPIs) that arise. However, modifying specific PPIs to effect deliberate changes to protein packing is an unmet challenge. In this work, we hypothesized that disrupting and replacing a highly conserved PPI with a DNA-DNA interaction would enable protein packing to be modulated by exploiting the programmability of the introduced oligonucleotides. Using concanavalin A (ConA) as a model protein, we circumvent potentially deleterious mutagenesis and exploit the selective binding of ConA toward mannose to noncovalently attach DNA to the protein surface. We show that DNA association eliminates the major PPI responsible for crystallization of native ConA, thereby allowing subtle changes to DNA design (length, complementarity, and attachment position) to program distinct changes to ConA packing, including the realization of three novel crystal structures and the deliberate expansion of ConA packing along a single crystallographic axis. These findings significantly enhance our understanding of how DNA can supersede native PPIs to program protein packing within ordered materials.