Intracellular protein delivery is an important tool for both therapeutic and fundamental applications. Effective protein delivery faces two major challenges: efficient cellular uptake and avoiding endosomal sequestration. We report here a general strategy for direct delivery of functional proteins to the cytosol using nanoparticle-stabilized capsules (NPSCs). These NPSCs are formed and stabilized through supramolecular interactions between the nanoparticle, the protein cargo, and the fatty acid capsule interior. The NPSCs are ~130 nm in diameter and feature low toxicity and excellent stability in serum. The effectiveness of these NPSCs as therapeutic protein carriers was demonstrated through the delivery of fully functional caspase-3 to HeLa cells with concomitant apoptosis. Analogous delivery of green fluorescent protein (GFP) confirmed cytosolic delivery as well as intracellular targeting of the delivered protein, demonstrating the utility of the system for both therapeutic and imaging applications.

Caspase-6 is an apoptotic cysteine protease that also governs disease progression in Huntington’s and Alzheimer’s Diseases. Caspase-6 is of great interest as a target for treatment of these neurodegenerative diseases, however the molecular basis of caspase-6 function and regulation remains poorly understood. In the recently reported structure of caspase-6, the 60’s and 130’s helices at the base of the substrate-binding groove extend upward, in a conformation entirely different from that of any other caspase. Presently, the central question about caspase-6 structure and function is whether the extended conformation is the catalytically competent conformation or whether the extended helices must undergo a large conformational rearrangement in order to bind substrate. We have generated a series of caspase-6 cleavage variants including a novel constitutively two-chain form and determined crystal structures of caspase-6 with and without the intersubunit linker. This series allows evaluation of the role of the prodomain and intersubunit linker on caspase-6 structure and function before and after substrate binding. Caspase-6 is inherently more stable than closely related caspases. Cleaved caspase-6 with both the prodomain and linker present is the most stable indicating that these two regions act in concert to increase stability, but maintain the extended conformation in the unliganded state. Most importantly, these data suggest that caspase-6 undergoes a significant conformational change upon substrate binding, adopting a structure that is more like canonical caspases.

Background: Caspase-6 is a critical factor in neurodegeneration, which is regulated by zinc binding. Results: Caspase-6 is inhibited by zinc and binds one zinc/monomer at an exosite distal from the active site. Conclusion: Zinc allosterically inhibits caspase-6 by locking it into a naturally occurring, inactive, and extended helical conformation. Significance: The allosteric inhibition observed in the presence of zinc may aid in the development of allosteric caspase-6 drugs.

Summary Caspases, a family of apoptotic proteases, are increasingly recognized as being extensively phosphorylated, usually leading to inactivation. To date, no structural mechanism for phosphorylation-based caspase inactivation is available, although this information may be key to achieving caspase-specific inhibition. Caspase-6 has recently been implicated in neurodegenerative conditions including Huntington's and Alzheimer's diseases. A full understanding of caspase-6 regulation is crucial to caspase-6-specific inhibition. Caspase-6 is phosphorylated by ARK5 kinase at serine 257 leading to suppression of cell death via caspase-6 inhibition. Our structure of the fully inactive phosphomimetic S257D reveals that phosphorylation results in a steric clash with P201 in the L2′ loop. Removal of the proline side chain alleviates the clash resulting in nearly wild-type activity levels. This phosphomimetic-mediated steric clash causes misalignment of the substrate-binding groove, preventing substrate binding. Substrate-binding loop misalignment appears to be a widely used regulatory strategy among caspases and may present a new paradigm for caspase-specific control.

One of the most promising and as yet underutilized means of regulating protein function is exploitation of allosteric sites. All caspases catalyze the same overall reaction, but they perform different biological roles and are differentially regulated. It is our hypothesis that many allosteric sites exist on various caspases and that understanding both the distinct and overlapping mechanisms by which each caspase can be allosterically controlled should ultimately enable caspase-specific inhibition. Here we describe the ongoing work and methods for compiling a comprehensive map of apoptotic caspase allostery. Central to this approach are the use of i) the embedded record of naturally evolved allosterically sites that are sensitive to zinc-medicated inhibition, phosphorylation and other post-translationally modifications, ii) structural and mutagenic approaches and iii) novel binding sites identified by both rationally-designed and screening-derived small-molecule inhibitors.

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