Cooperative, reversible self-assembly of covalently pre-linked proteins into giant fibrous structures.

We demonstrate a simple bioconjugate polymer system that undergoes reversible self-assembling into extended fibrous structures, reminiscent of those observed in living systems. It is comprised of green fluorescent protein (GFP) molecules linked into linear oligomeric strands through click step growth polymerization with dialkyne poly(ethylene oxide) (PEO). Confocal microscopy, atomic force microscopy, and dynamic light scattering revealed that such strands form high persistence length fibers, with lengths reaching tens of micrometers, and uniform, sub-100 nm widths. We ascribe this remarkable and robust form of self-assembly to the cooperativity arising from the known tendency of GFP molecules to dimerize through localized hydrophobic patches and from their covalent pre-linking with flexible PEO. Dissipative particle dynamics simulations of a coarse-grained model of the system revealed its tendency to form elongated fibrous aggregates, suggesting the general nature of this mode of self-assembly.

Efficient synthesis and in vivo incorporation of acridon-2-ylalanine, a fluorescent amino acid for lifetime and Förster resonance energy transfer/luminescence resonance energy transfer studies.

The amino acid acridon-2-ylalanine (Acd) can be a valuable probe of protein conformational change because it is a long lifetime, visible wavelength fluorophore that is small enough to be incorporated during ribosomal biosynthesis. Incorporation of Acd into proteins expressed in Escherichia coli requires efficient chemical synthesis to produce large quantities of the amino acid and the generation of a mutant aminoacyl tRNA synthetase that can selectively charge the amino acid onto a tRNA. Here, we report the synthesis of Acd in 87% yield over five steps from Tyr and the identification of an Acd synthetase by screening candidate enzymes previously evolved from Methanococcus janaschii Tyr synthetase for unnatural amino acid incorporation. Furthermore, we characterize the photophysical properties of Acd, including quenching interactions with select natural amino acids and Förster resonance energy transfer (FRET) interactions with common fluorophores such as methoxycoumarin (Mcm). Finally, we demonstrate the value of incorporation of Acd into proteins, using changes in Acd fluorescence lifetimes, Mcm/Acd FRET, or energy transfer to Eu(3+) to monitor protein folding and binding interactions.

A protein-polymer hybrid mediated by DNA.

Protein-polymer hybrids (PPHs) represent an important and rapidly expanding class of biomaterials. Typically in these hybrids the linkage between the protein and the polymer is covalent. Here we describe a straightforward approach to a noncovalent PPH that is mediated by DNA. Although noncovalent, the DNA-mediated approach affords the highly specific pairing and assembly properties of DNA. To obtain the protein-DNA conjugate for assembly of the PPH, we report here the first direct copper catalyzed azide-alkyne cycloaddition-based protein-DNA conjugation. This significantly simplifies access to protein-DNA conjugates. The protein-DNA conjugate and partner polymer-DNA conjugate are readily assembled through annealing of the cDNA strands to obtain the PPH, the assembly of which was confirmed via dynamic light scattering and fluorescence spectroscopy.

Genetically encoded initiator for polymer growth from proteins.

Despite the importance of protein-polymer bioconjugates, there is no general method for producing homogeneous recombinant protein that contains polymer initiators at defined sites. To address this deficiency, we designed the amino acid 4-(2'-bromoisobutyramido)phenylalanine (1) as an initiator in atom-transfer radical polymerization (ATRP) that would provide a stable linkage between the protein and growing polymer. We synthesized 1 and evolved a Methanococcus jannaschii tyrosyl-tRNA synthetase/tRNA(CUA) pair to genetically encode this initiator in response to an amber codon. To demonstrate the utility of this initiator, we produced green fluorescent protein (GFP) with 1 site-specifically incorporated on its surface (GFP-1). Purified GFP-1 was then used as an initiator under standard ATRP conditions with a monomer, oligo(ethylene oxide) monomethyl ether methacrylate, efficiently producing a polymer-GFP bioconjugate where the polymer is connected at our selected site on GFP.