Metal-mediated self-assembly of protein superstructures: influence of secondary interactions on protein oligomerization and aggregation

Controlled protein dimerization through hybrid coordination motifs

Protein homodimerization is the simplest form of oligomerization that is frequently utilized for the construction of functional biological assemblies and the regulation of cellular pathways. Despite its simplicity, dimerization still poses an enormous challenge for protein engineering and chemical manipulation, owing to the large molecular surfaces involved in this process. We report here the construction of a hybrid coordination motif--consisting of a natural (His) and a non-natural ligand (quinolate)--on the alpha-helical surface of cytochrome cb(562), which (a) simultaneously binds divalent metals with high affinity, (b) leads to a metal-induced increase in global protein stability, and importantly, (c) enables the formation of a discrete protein dimer, whose shape is dictated by the inner-sphere metal coordination geometry and closely approximates that of the DNA-binding domains of bZIP family transcription factors.

Evolution of metal selectivity in templated protein interfaces

Selective binding by metalloproteins to their cognate metal ions is essential to cellular survival. How proteins originally acquired the ability to selectively bind metals and evolved a diverse array of metal-centered functions despite the availability of only a few metal-coordinating functionalities remains an open question. Using a rational design approach (Metal-Templated Interface Redesign), we describe the transformation of a monomeric electron transfer protein, cytochrome cb(562), into a tetrameric assembly ((C96)RIDC-1(4)) that stably and selectively binds Zn(2+) and displays a metal-dependent conformational change reminiscent of a signaling protein. A thorough analysis of the metal binding properties of (C96)RIDC-1(4) reveals that it can also stably harbor other divalent metals with affinities that rival (Ni(2+)) or even exceed (Cu(2+)) those of Zn(2+) on a per site basis. Nevertheless, this analysis suggests that our templating strategy simultaneously introduces an increased bias toward binding a higher number of Zn(2+) ions (four high affinity sites) versus Cu(2+) or Ni(2+) (two high affinity sites), ultimately leading to the exclusive selectivity of (C96)RIDC-1(4) for Zn(2+) over those ions. More generally, our results indicate that an initial metal-driven nucleation event followed by the formation of a stable protein architecture around the metal provides a straightforward path for generating structural and functional diversity.

Metal-directed protein self-assembly

Proteins are nature's premier building blocks for constructing sophisticated nanoscale architectures that carry out complex tasks and chemical transformations. Some 70%-80% of all proteins are thought to be permanently oligomeric; that is, they are composed of multiple proteins that are held together in precise spatial organization through noncovalent interactions. Although it is of great fundamental interest to understand the physicochemical basis of protein self-assembly, the mastery of protein-protein interactions (PPIs) would also allow access to novel biomaterials with nature's favorite and most versatile building block. In this Account, we describe a new approach we have developed with this possibility in mind, metal-directed protein self-assembly (MDPSA), which utilizes the strength, directionality, and selectivity of metal-ligand interactions to control PPIs. At its core, MDPSA is inspired by supramolecular coordination chemistry, which exploits metal coordination for the self-assembly of small molecules into discrete, more-or-less predictable higher order structures. Proteins, however, are not exactly small molecules or simple metal ligands: they feature extensive, heterogeneous surfaces that can interact with each other and with metal ions in unpredictable ways. We begin by first describing the challenges of using entire proteins as molecular building blocks. We follow with an examination of our work on a model protein (cytochrome cb(562)), highlighting challenges toward establishing ground rules for MDPSA as well as progress in overcoming these challenges. Proteins are also nature's metal ligands of choice. In MDPSA, once metal ions guide proteins into forming large assemblies, they are by definition embedded within extensive interfaces formed between protein surfaces. These complex surfaces make an inorganic chemist's life somewhat difficult, yet they also provide a wide platform to modulate the metal coordination environment through distant, noncovalent interactions, exactly as natural metalloproteins and enzymes do. We describe our computational and experimental efforts toward restructuring the noncovalent interaction network formed between proteins surrounding the interfacial metal centers. This approach, of metal templating followed by the redesign of protein interfaces (metal-templated interface redesign, MeTIR), not only provides a route to engineer de novo PPIs and novel metal coordination environments but also suggests possible parallels with the evolution of metalloproteins.

Metal templated design of protein interfaces

Metal coordination is a key structural and functional component of a large fraction of proteins. Given this dual role we considered the possibility that metal coordination may have played a templating role in the early evolution of protein folds and complexes. We describe here a rational design approach, Metal Templated Interface Redesign (MeTIR), that mimics the time course of a hypothetical evolutionary pathway for the formation of stable protein assemblies through an initial metal coordination event. Using a folded monomeric protein, cytochrome cb(562), as a building block we show that its non-self-associating surface can be made self-associating through a minimal number of mutations that enable Zn coordination. The protein interfaces in the resulting Zn-directed, D(2)-symmetrical tetramer are subsequently redesigned, yielding unique protein architectures that self-assemble in the presence or absence of metals. Aside from its evolutionary implications, MeTIR provides a route to engineer de novo protein interfaces and metal coordination environments that can be tuned through the extensive noncovalent bonding interactions in these interfaces.

A superprotein triangle driven by nickel(II) coordination: exploiting non-natural metal ligands in protein self-assembly

We previously devised a strategy (metal-directed protein self-assembly, MDPSA) that utilizes the simultaneous stability, lability, and directionality of metal-ligand bonds to drive protein-protein interactions. Here we show that both the structural and functional scopes of MDPSA can be broadened by incorporation of non-natural metal-chelating ligands onto protein surfaces. A cytochrome cb(562) variant, MBP-Phen1, which features a covalently attached phenanthroline (Phen) group on its surface, self-assembles into an unusual triangular architecture (Ni(3):MBP-Phen1(3)) upon binding Ni as a result of specific Phen-protein interactions. The crystal structure of Ni(3):MBP-Phen1(3) reveals that the Phen group is buried in a small pocket on the protein surface, which results in an unsaturated Ni coordination environment.

Control of protein oligomerization symmetry by metal coordination: C2 and C3 symmetrical assemblies through Cu(II) and Ni(II) coordination

We describe the metal-dependent self-assembly of symmetrical protein homooligomers from protein building blocks that feature appropriately engineered metal-chelating motifs on their surfaces. Crystallographic studies indicate that the same four-helix-bundle protein construct, MBPC-1, can self-assemble into C(2) and C(3) symmetrical assemblies dictated by Cu(II) and Ni(II) coordination, respectively. The symmetry inherent in metal coordination can thus be directly applied to biological self-assembly.