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Antibodies are indispensable tools in protein engineering and structural biology. Antibodies suitable for structural studies should recognize the 3-dimensional (3D) conformations of target proteins. Generating such antibodies and characterizing their complexes with antigens take a significant amount of time and effort. Here, we show that we can expand the application of well-characterized antibodies by “transplanting” the epitopes that they recognize to proteins with completely different structures and sequences. Previously, several antibodies have been shown to recognize the alpha-helical conformation of antigenic peptides. We demonstrate that these antibodies can be made to bind to a variety of unrelated “off-target” proteins by modifying amino acids in the preexisting alpha helices of such proteins. Using X-ray crystallography, we determined the structures of the engineered protein–antibody complexes. All of the antibodies bound to the epitope-transplanted proteins, forming accurately predictable structures. Furthermore, we showed that binding of these antihelix antibodies to the engineered target proteins can modulate their catalytic activities by trapping them in selected functional states. Our method is simple and efficient, and it will have applications in protein X-ray crystallography, electron microscopy, and nanotechnology.

Hemagglutinin (HA) displayed on a ferritin nano-cage has been shown to be effective in generating a potent immune response against a broad range of influenza infections. Here, we showed that conjugation of flagellin together with HA to the exterior surface of the ferritin cage greatly enhanced not only the humoral immune response in mice but also antigen-specific T cell responses that include Th1 cytokine secretion. The effect of flagellin remained essentially unchanged when the molar ratio of flagellin to HA was reduced from 1:1 to 1:3. Injection of the ferritin-HA-flagellin cage provided protection against lethal virus challenge in mice. We used a small immunoglobulin fragment VL12.3 as a convenient method for attaching HA and flagellin to the ferritin cage. This attachment method can be used for rapid screening of a variety of protein cages and nano-assemblies to identify the most suitable carrier and adjuvant proteins for the target antigen.

The activity and morphology of mitochondria are maintained by dynamic fusion and fission processes regulated by a group of proteins residing in, or attached to, their inner and outer membranes. Hypoxia-induced gene domain protein-1a (Higd-1a)/HIMP1-a/HIG1, a mitochondrial inner membrane protein, plays a role in cell survival under hypoxic conditions. In the present study, we showed that Higd-1a depletion resulted in mitochondrial fission, depletion of mtDNA, disorganization of cristae, and growth retardation. We demonstrated that Higd-1a functions by specifically binding to Optic atrophy 1 (Opa1), a key element in fusion of the inner membrane. In the absence of Higd-1a, Opa1 was cleaved, resulting in the loss of its long isoforms and accumulation of small soluble forms. The small forms of Opa1 do not interact with Higd-1a, suggesting that a part of Opa1 in or proximal to the membrane is required for that interaction. Opa1 cleavage, mitochondrial fission, and cell death induced by dissipation of the mitochondrial membrane potential were significantly inhibited by ectopic expression of Higd-1a. Furthermore, growth inhibition due to Higd-1a depletion could be overcome by overexpression of a noncleavable form of Opa1. Collectively, our observations demonstrate that Higd-1a inhibits Opa1 cleavage and is required for mitochondrial fusion by virtue of its interaction with Opa1.