Molecular probes for cellular imaging of post-translational proteoforms

Intracellular delivery of proteins for live cell imaging

The majority of cellular functions are regulated by intracellular proteins, and regulating their interactions can unlock fundamental insights in biology and open new avenues for drug discovery. Because the vast majority of intracellular targets remain undruggable, there is significant current interest in developing protein-based agents especially monoclonal antibodies due to their specificity, availability, and established screening/engineering methods. However, efficient delivery of proteins into the cytoplasm has been a major challenge in biological engineering and drug discovery. We previously reported a platform technology based on a Coomassie blue-cholesterol conjugate (CB-tag) capable of delivering small proteins directly into the cytoplasm. Here, we report a new generation of CB-tag that can bring proteins with a wide size range into the cytoplasm, bypassing endosomal sequestration. Remarkably, intracellular targets with distinct structures were visualized. Overall, the new CB-tag demonstrated a robust ability in protein delivery with broad applications ranging from live-cell immunofluorescence to protein-based therapeutic development.

Site-Specific and Fluorescently Enhanced Installation of Post-Translational Protein Modifications via Bifunctional Biarsenical Linker

To understand how particular post-translational modifications (PTMs) affect the function of a target protein, it is essential to first prepare and investigate the target with the modification at the desired position. This drives the continuous development of site-specific protein modification technologies. Here, we present the chemical synthesis and application of the biarsenical linker SrtCrAsH-EDT2, which has a dual labeling functionality. This linker, containing a sortase A recognition motif, can be conjugated with any protein containing the LPXTG motif at the C terminus, such as ubiquitin and the SUMO tag, and then attached to a protein of interest (POI) containing a terminal or bipartite (intramolecularly placed) tetracysteine motif. This modification of the POI facilitates the straightforward and rapid incorporation of PTMs, which are further highlighted by the fluorescent biarsenical probe. Consequently, this directly correlates proteins' physical properties and cellular roles under various physiological conditions or in disease states. The proposed one-pot labeling methodology can be utilized to explore the effects of PTMs on proteins, affecting their structure, function, localization, and interactions within the cellular environment. Understanding these effects is crucial for uncovering the complex mechanisms that regulate cellular function and dysfunction.

Synthetic Proteins behind the Plasma Barrier: Molecular Spies

There is a continuous demand to improve our understanding of fundamental processes that underlie human health and disease. Therefore, novel strategies that can assist in these efforts are required. For example, molecular biology and genetic approaches have revolutionized our understanding of protein-mediated processes by facilitating their direct visualization and analyses in living cells. Despite these developments, genetic manipulation has limitations in controlling events that occur after translation such as posttranslational modifications (PTMs), which are imperative regulatory elements. As a result, developing new methods to study PTMs in live cells is a major bottleneck in deciphering their exact roles in the myriad cellular processes.

Synthetic and semisynthetic proteins are prepared by combining solid phase peptide synthesis (SPPS) and chemoselective ligation approaches with synthetic or recombinant peptides. Employing protein synthesis allows chemists to incorporate natural and unnatural modifications with virtually unlimited number of functional groups into the protein’s sequence, such as PTMs and their mimics. In addition, synthetic proteins can include additional elements such as fluorescent tags, reactive groups, caged units, and enrichment handles. Therefore, harnessing the power of chemical protein synthesis offers great opportunities to study fundamental biological processes.

Unfortunately, the low cell permeability of proteins limits their applications mainly to in vitro settings, excluding live cell studies. As a result, chemical biologists have been attempting to overcome these limitations by developing protein delivery methods that would enable the study of custom-made proteins in a biological context. Success with these strategies should enable accurate determination of protein localization, degradation, folding, interactions, and involvement in the assembly of membrane-less organelles formed by liquid–liquid phase separation inside cells. Importantly, protein delivery approaches are complementary to genetic manipulations, and combining these approaches should pave the way to new discoveries.

In this Account, we describe recent developments in protein delivery methods, with emphasis on those most compatible with synthetic proteins. We highlight experimental approaches and conceptual adaptations required to design and study synthetic proteins in live cells, with or without genetic manipulation. In addition, we highlight the strength and weakness of these approaches for both the delivery and the subsequent studies. We also describe our endeavors to deliver synthetic proteins to cells via cell penetrating peptides (CPPs) and multiplexed bead loading (MBL), as showcases of the applications of these methods to shed light on biological processes. Lastly, we contemplate other future applications of synthetic proteins to answer questions that are currently unapproachable.