Design of self-assembling peptide hydrogelators amenable to bacterial expression

Hydrogels formed from self-assembling peptides are finding use in tissue engineering and drug delivery applications. Given the notorious difficulties associated with producing self-assembling peptides by recombinant expression, most are typically prepared by chemical synthesis. Herein, we report the design of a family of self-assembling β-hairpin peptides amenable to efficient production using an optimized bacterial expression system. Expressing peptides, EX1, EX2 and EX3 contain identical eight-residue amphiphilic β-strands connected by varying turn sequences that are responsible for ensuring chain reversal and the proper intramolecular folding and consequent self-assembly of the peptide into a hydrogel network under physiological conditions. EX1 was initially used to establish and optimize the bacterial expression system by which all the peptides could be eventually individually expressed. Expression clones were designed to allow exploration of possible fusion partners and investigate both enzymatic and chemical cleavage as means to liberate the target peptide. A systematic analysis of possible expression systems followed by fermentation optimization lead to a system in which all three peptides could be expressed as fusions with BAD-BH3, the BH3 domain of the proapoptotic BAD (Bcl-2 Associated Death) Protein. CNBr cleavage followed by purification afforded 50, 31, and 15 mg/L yields of pure EX1, EX2 and EX3, respectively. CD spectroscopy, TEM, and rheological analysis indicate that these peptides fold and assembled into well-defined fibrils that constitute hydrogels having shear-thin/recovery properties.