Decoding repetitive proteins to engineer stimuli-responsive biopolymers

Danielle J. Mai, PhD
Stanford University

Abstract: Evolution allows organisms to adapt to their environments, yet some molecular patterns stay constant through billions of years of evolution. These patterns encode the secrets of biological materials, such that similar patterns often emerge in different materials with similar functions. For example, a family of elastin proteins form stretchable fibers, which allow the constant movement of human skin, blood vessels, and lungs. To decode these secrets, we look for patterns in the sequences of proteins with similar functions. These conserved sequences often emerge from repetitive regions, and “consensus repeat sequences” provide a convenient platform to investigate protein sequence–biomaterial property relationships.

Here, we explore two classes of repetitive proteins as stimuli-responsive biopolymers. First, we investigate the ion-responsive behavior of repetitive proteins that undergo conformational changes in response to calcium ions. Calcium ions trigger numerous biological phenomena including bone growth, muscle contraction, and neurotransmitter release. We Modify Hydrophobicity, electrostatics, and sequence heterogeneity of calcium-responsive proteins to demonstrate sequence-dependent, reversible folding in the presence of calcium ions by circular dichroism, as well as domain size changes by small-angle X-ray scattering. Hydrogels comprising calcium-responsive proteins reveal the impact of sequence on hydrogel stability, calcium sensitivity, shear modulus, and characteristic relaxation time. Second, we introduce an enzymatic stimulus to drive isothermal phase separation of elastin-like polypeptides (ELPs). ELPs have drawn broad interest as thermally responsive biopolymers, such that ELPs undergo reversible phase separation from water upon heating. However, temperature is not always a convenient trigger, especially in biological environments that maintain near-constant temperature, pH, and ionic composition. To expand ELP function, we program a mechanism for isothermal phase separation upon proteolytic cleavage of a di-block ELP. Proteolytic cleavage presents a new opportunity to exploit isothermal, biological triggers for ELP phase behavior and self-assembly processes. Overall, we demonstrate repetitive proteins as tunable and modular building blocks for functional biomaterials.