Intracellular Delivery of Peptides and Proteins with an Engineered Membrane Translocation Domain

Antibodies and other protein therapeutics have revolutionized medicine, but their application is largely limited to extracellular targets. The lack of efficient intracellular delivery methods remains a major bottleneck. Here, we engineered a family of small (~90 amino acids), metabolically stable membrane translocation domains (MTDs) by modifying the loop sequences of a human fibronectin type III (FN3) domain. The most potent variant, MTD4, is highly cell-permeable and can be recombinantly fused to the N- or C-terminus of any peptide or protein, serving as a versatile "plug-and-play" vehicle. We demonstrate that MTD4 fusions efficiently deliver a wide variety of functional peptides and proteins into the cytosol and nucleus of eukaryotic cells, both <i>in vitro</i> and <i>in vivo</i>. Following systemic administration, MTD4 fusion proteins exhibit broad biodistribution and homogenous tissue penetration in mice. Importantly, MTD4 is effective at low nanomolar (nM) concentrations, making it a promising platform for addressing a vast range of intracellular and previously "undruggable" targets.