Caspase‐6 Self‐Activation Enables Distinct Helix‐Strand Interconversion Upon Substrate Binding

Caspases are cysteine proteases that are major players in key cellular processes, including apoptosis and inflammation. Amongst the human caspases, caspase‐6 has been implicated in playing a unique and critical role in the neurodegenerative pathways of Alzheimer's, Huntington's and Parkinson's disease. Unfortunately, structural similarities between caspase‐6 and other caspases have hampered precise targeting of caspase‐6 uniquely. All caspases can exist in a canonical conformation, in which the substrate binds atop a beta‐strand platform in the 130's region. This caspase‐6 region can also adopt a helical conformation that has not been seen in any other caspases. We have shown that caspase‐6 is inherently and dramatically more conformationally dynamic than closely related caspase‐7. In contrast to caspase‐7, which rests constitutively in the strand conformation before and after substrate binding, hydrogen/deuterium exchange data for the L2’ and 130's regions suggested that prior to substrate binding, caspase‐6 exists in a dynamic equilibrium between the helix and strand conformations. Caspase‐6 transitions exclusively to the canonical strand conformation only upon substrate binding. Glu‐135, which showed a noticeably different calculated pK a s in the helix and strand conformations, appears to play a key role in the interconversion between the helix and strand conformations. We have also mapped the local changes in the conformational flexibility of procaspase‐6 at the discrete states that reflect series of cleavage events that ultimately lead to the fully active, substrate‐bound state. Intramolecular self‐cleavage at Asp‐193 evoked higher solvent exposure in the regions of the substrate binding loops L1, L3, L4 and in the 130's, the intersubunit linker region, and the 26–32 region, as well as stabilized loop 2. Further removal of the linker allowed caspase‐6 to gain more flexibility in the 130's region and in the L2 region converting caspase‐6 to a competent substrate‐binding state. The prodomain region was found to be intrinsically disordered independent of the activation step of caspase‐6; however, its complete removal resulted in the protection of the adjacent 26–32 region, suggesting that this region may play a regulatory role, likely as an exosite for substrate recognition. The molecular details of caspase‐6 dynamics in solution provide a comprehensive scaffold for strategic design of therapeutic approaches for neurodegenerative disorders. Support or Funding Information This work was supported by the National Institutes of Health (NIH) GM 080532. This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .