Structural basis of calcineurin activation by calmodulin

Area of Science:

• Molecular Biology
• Structural Biology
• Biochemistry

Background:

• Calcineurin is a critical calmodulin (CaM)-activated protein phosphatase regulating cellular processes and calcium signaling.
• The mechanism of CaM-mediated calcineurin activation, specifically how CaM displaces the autoinhibitory domain (AID), remains structurally undefined.

Purpose of the Study:

• To elucidate the structural basis of calcineurin activation by calmodulin.
• To investigate the role of the autoinhibitory domain in CaM-dependent calcineurin activity.

Main Methods:

• Creation of a fused ternary complex (CBA) linking CaM, calcineurin regulatory B subunit (CnB), and calcineurin catalytic A subunit (CnA).
• Biochemical assays to compare CBA activity with native calcineurin.
• X-ray crystallography to determine the structure of the CBA complex.

Main Results:

• The fused CBA complex exhibits catalytic activity comparable to fully activated native calcineurin.
• Crystal structure reveals no significant change in the active site and no direct CaM interaction in the ordered structure.
• CaM binding displaces the disordered segment of the AID, which adopts an alpha-helical conformation upon binding.

Conclusions:

• Calcineurin activation by CaM is mediated by the displacement of the disordered AID fragment, thereby granting access to the active site.
• This mechanism explains how CaM binding overcomes autoinhibition to enable calcineurin's enzymatic function in calcium signaling pathways.