A Ca(2+) switch aligns the active site of calpain

Area of Science:

• Biochemistry
• Molecular Biology
• Cell Biology

Background:

• Calcium (Ca2+) signaling regulates calpains, proteases crucial for cellular functions like cytoskeleton remodeling.
• Dysregulated calcium levels and calpain activity are implicated in tissue damage (ischemia) and neurodegeneration (Alzheimer's disease).

Purpose of the Study:

• To elucidate the calcium-binding mechanism essential for activating the protease core of mu-calpain.
• To investigate the evolutionary conservation of calpain activation across different isoforms.

Main Methods:

• X-ray crystallography at 2.1 A resolution to determine the structure of the mu-calpain protease core.
• Analysis of conserved calcium-binding residues across various calpain isoforms.

Main Results:

• The crystal structure revealed that activating the mu-calpain protease core requires the cooperative binding of two Ca(2+) ions at two novel non-EF-hand sites.
• Conservation of these Ca(2+) binding residues suggests a general, ancestral activation mechanism for most calpains, including those without EF-hand domains.
• The inhibitor calpastatin does not affect the protease region, indicating its potential role in pathologies upon autoproteolytic release.

Conclusions:

• A conserved, two-calcium-ion binding mechanism activates the calpain protease core, providing a unified view of calpain activation.
• This mechanism is fundamental to calpains, extending to isoforms lacking canonical EF-hand domains.
• The structural insights into calpain activation and its independence from calpastatin may inform strategies for treating calpain-associated diseases.