Abstract: Provided are systems and methods for tagging biomolecules of interest, methods of generating tagged biomolecules, and methods of use thereof. An adapter system comprises one or more epitope tags operably linked to an antibody binding domain (AbBD), which may be a photoreactive antibody binding domain (pAbBD). A pAbBD may be used to covalently photocrosslink epitope tags to immunoglobulins or immunoglobulin fragments, which may in turn be used to label biomolecules of interest. Such epitope-tagged immunoglobulins may be used in various biological research applications, including multiplexed staining assays and tandem affinity pulldown assays.

Abstract: The invention relates to the use of precursor peptides and B1 proteins from lasso peptide biosynthesis systems for affinity purification, proximity-based sortase-mediated protein purification and ligation, and detection of fusion proteins. For proximity-based sortase-mediated protein purification and ligation, the invention relates to techniques that link protein purification with conjugation to other agents, including therapeutic agents, imaging agents, or linkers.

Abstract: The present disclosure provides compositions and methods for site-specific labeling of antibodies by proximity-based sortase-mediated ligation. The ligation method utilizes a non-canonical isopeptide ligation reaction catalyzed by newly identified variants of S. aureus sortase A. An antibody binding domain (e.g., protein A or protein G) is fused to the variants of SrtA to bring the enzyme into close proximity of an antibody, thereby significantly increases the efficiency of isopeptide bond formation.

Abstract: This invention relates to the compositions and methods for labeling antibodies and other proteins and targeting agents with a helical bundle protein that is functionalized with cargo. Cargos can include but are not limited to fluorescent dyes, haptens (e.g. biotin), contrast agents (e.g. gadolinium, radionuclides), chelated metals, therapeutic agents, sensitizers, or other small molecules. Specifically, provided herein are compositions having a helical bundle, that has been labeled at precisely defined locations with cargo, and that can be conjugated, attached or fused to an antibody or other targeting agent.