Design of helical linkers for fusion proteins and protein-based nanostructures.

The construction of recombinant fusion/chimeric proteins has been widely used for expression of soluble proteins and protein purification in a variety of fields of protein engineering and biotechnology. Fusion proteins are constructed by the linking of two protein domains with a peptide linker. The selection of a linker sequence is important for the construction of stable and bioactive fusion proteins. Empirically designed linkers are generally classified into two categories according to their structural features: flexible linkers and rigid linkers. Rigid linkers with the α-helix-forming sequences A(EAAAK)A (n=2-5) were first designed about two decades ago to control the distance between two protein domains and to reduce their interference. Thereafter, the helical linkers have been applied to the construction of many fusion proteins to improve expression and bioactivity. In addition, the design of fusion proteins that self-assemble into supramolecular complexes is useful for nanobiotechnology and synthetic biology. A protein that forms a self-assembling oligomer was fused by a rigid helical linker to another protein that forms another self-assembling oligomer, and the fusion protein symmetrically self-assembled into a designed protein nanoparticle or nanomaterial. Moreover, to construct chain-like polymeric nanostructures, extender protein nanobuilding blocks were designed by tandemly fusing two dimeric de novo proteins with helical or flexible linkers. The linker design of fusion proteins can affect conformation and dynamics of self-assembling nanostructures. The present review and methods focus on useful helical linkers to construct bioactive fusion proteins and protein-based nanostructures.