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.