Protein self-assembly is a particularly attractive option for bottom-up creation of tunable smart materials and hybrid inorganic/organic functional devices. Many disparate proteins naturally self assemble into discrete structures such as viral capsids or long filaments. Proteins possess intrinsic functionality and the ability to modify or append structures genetically. Furthermore, display technologies have unleashed a plethora of small peptide sequences that bind to a diverse range of materials or form a wide range of inorganic crystals. Therefore, proteins can potentially serve as scaffolds for a host of nanostructured materials by self-assembling them at precise spatial distances and directing their placement onto solid supports at defined locations.

We wish to find a general approach to build protein assemblages of arbitrary geometry over multiple length scales. To achieve this goal, the following benchmarks must be overcome: (A) a suitable protein template must be identified. (B) Control of the length of the protein template must be demonstrated. (C) Genetic modification of the template must be established. (D) Absolute control over a single dimension (e.g., length) must be engineered. These initial four steps directly lead into creation of more complex shapes and geometries. This presentation elucidates the progress we have made in these areas.

Protein filaments are exciting starting templates since the filaments demonstrate assembly over one dimension. We have discovered and characterized a unique filamentous protein, γ prefoldin (γ PFD), from the hyperthermophile Methanocaldococcus jannaschii. The filament lengths are polydisperse and exceed 1 micron; the purported linear structure consists of a single repeating beta strand scaffold with protruding coiled-coils orthogonal to the direction of the filament length. Both the extraordinary stability of the γ PFD at temperatures in excess of 97oC and the distinct modular architecture of the filaments make for an effective starting scaffold for subsequent protein assembly and nanotechnology applications.

We present the rational design of a γ PFD variant that caps the end of the filament over length scales from less than 10 to over 100 nm. We then demonstrate tunability of the filament length and evaluate different principal parameters for filament formation such as stability, rates of formation, and structural integrity. A predictive model of filament formation provided the kinetic parameters that govern filament assembly and showed quantitative agreement with the experimental filament distribution lengths. This knowledge was used to design a series of filament variants containing active peptide fusion partners in tight distributions spanning a range of filament lengths.

We conclude with a discussion on recent attempts to design absolute control over the filament length by re-engineering the protein-protein interaction sites on γ PFD. We describe the creation of specifically interacting proteins for ordered architectures through the application of powerful genetic selection technology. These engineered filaments are to be used as building blocks for 3-D architectures of arbitrary geometry, and progress towards this end will be discussed.