Conformationally constrained sequence designs to bias monomer-dimer equilibriums in TASP systems

We have designed template-assembled synthetic proteins (TASPs) with the intent of controlling their oligomeric state by stabilizing specific helical tertiary structures via histidine metal ion chelation or disulfide incorporation. In solution, cavitein Q4 was previously determined to interconvert between a four-helix bundle monomer and an eight-helix bundle dimer. In this paper, we show that judicious mutation of cavitein Q4 can stabilize either the monomeric parallel four-helix bundle or the dimeric antiparallel eight-helix bundle structure. Cavitein Q4-E3H, designed to be dimeric, is indeed biased toward dimerization as a result of incorporation of histidines. Moreover, the addition of nickel was found to further increase the association constant of dimerization. Similarly, a cavitein designed to stabilize the monomeric structure via histidine metal ion chelation (Q4-H) was found to favor a monomer in solution upon addition of nickel. Lastly, a cavitein intended to stabilize a monomeric structure via disulfide incorporation (Q4-C2) is reported. Surprisingly, this disulfide cavitein yielded two products upon oxidation suggesting disulfide formation both above the cavitand template and below may be possible. Nevertheless, the two disulfide caviteins were shown to exist as monomers as per their design.