Calcineurin regulatory subunit B is a unique calcium sensor that regulates calcineurin in both calcium-dependent and calcium-independent manner.

Calcineurin subunit B (CNB), a regulatory subunit of calcineurin, is a member of EF-hand calcium (Ca(2+))-binding protein superfamily. In this study, we performed phosphatase activity, pull-down, and circular dichroism analyses, which shows Ca(2+)-free CNB (apo-CNB) is able to bind CNA (Calcineurin subunit A) and regulate its phosphatase activity, albeit to a lesser extent than Ca(2+)-saturated CNB (holo-CNB). This is supported by subsequent molecular dynamics (MD) simulations which aimed to examine calcium-induced conformational changes of CNB, using both holo-CNB and apo-CNB. Based on the trajectories of MD, we found that there were no drastic changes between holo-CNB and apo-CNB. The modest differences seen are mainly reflected in the interhelical angles of the four EF-hand motifs and solvent accessible surface area of the hydrophobic groove which is responsible for CNA binding. Interestingly, this hydrophobic groove was largely retained in both holo-CNB and apo-CNB. Comparative analyses show that the calcium-induced conformational change in CNB is remarkably different from that of the two prototypical EF-hand calcium-binding proteins, namely, calmodulin and recoverin. This is the first time that CNB is shown to activate CNA in the absence of Ca(2+). 2009 Wiley-Liss, Inc.