Abstract
Streptavidin (SA) and biotin have the strongest known binding interaction in nature, with a KD in the
femtomolar range (4.8x10-14 M). This extraordinary binding affinity has led to its ubiquitous use in biomedical
research and diagnostics. Though SA/biotin enjoys success in many applications, abundant endogenous biotin
attenuates assay sensitivity. Moreover, SA is a highly immunogenic foreign protein, which limits its use in
therapeutic applications such as pretargeted immunotherapy (PTI).
Mirror-image SA and biotin (D-SA/L-biotin) offer an elegant solution to these problems. For example, D-
Proteins are inert to L-proteases and therefore cannot be digested for MHC presentation to the immune system.
This property means that D-SA will have greatly decreased immunogenicity and increased half-life compared to
L-SA. Additionally, symmetry dictates that the mirror-image pair (D-SA/L-biotin) will have the exact same
exceptional affinity as the natural pair (L-SA/D-biotin). Importantly, we have discovered that D-biotin has minimal
binding to D-SA. Therefore, we propose that D-SA/L-biotin can be used as a biotin orthogonal streptavidin system
(BOSS). We hypothesize that the orthogonality of BOSS, along with D-SA’s low immunogenicity, will
overcome the limitations of natural SA/biotin.
We will chemically synthesize D-SA using solid-phase peptide synthesis with D-amino acids and native
chemical ligation. We will then replicate a previously reported SA PTI method using BOSS. We will attach this
D-SA to the C-terminus of the antibody fragment (scFv) used in the previous study (scFv-D-SA) and will
recombinantly express its L-counterpart (scFv-L-SA). This scFv binds to GD2, a cell-surface glycosphingolipid
that is upregulated in neuroblastoma (NB). We will first measure the efficacy of scFv-D-SA in NB cells using
fluorescence microscopy. We will attach a red fluorophore to D-biotin and magenta fluorophore to L-biotin to
determine the relative binding of scFv-D-SA and scFv-L-SA to the NB cells and a panel of control cells. We will
then test our BOSS PTI method in mice xenografted with NB cells using fluorescence-based full-body imaging
to look for enhanced fluorescence localized around the tumor. We expect that BOSS will dramatically improve
current pretargeting efforts. Moreover, given the ubiquity of biotin/SA-based systems throughout biomedical
science, we also expect BOSS to be widely applicable and relevant to proximity labeling, diagnostic testing, and
any method that suffers from SA immunogenicity and biotin interference.