Identification of the critical structural determinants of the EF-hand domain arrangements in calcium binding proteins.

EF-hand calcium binding proteins (CaBPs) share strong sequence homology, but exhibit great diversity in structure and function. Thus although calmodulin (CaM) and calcineurin B (CNB) both consist of four EF hands, their domain arrangements are quite distinct. CaM and the CaM-like proteins are characterized by an extended architecture, whereas CNB and the CNB-like proteins have a more compact form. In this study, we performed structural alignments and molecular dynamics (MD) simulations on 3 CaM-like proteins and 6 CNB-like proteins, and quantified their distinct structural and dynamical features in an effort to establish how their sequences specify their structures and dynamics. Alignments of the EF2-EF3 region of these proteins revealed that several residues (not restricted to the linker between the EF2 and EF3 motifs) differed between the two groups of proteins. A customized inverse folding approach followed by structural assessments and MD simulations established the critical role of these residues in determining the structure of the proteins. Identification of the critical determinants of the two different EF-hand domain arrangements and the distinct dynamical features relevant to their respective functions provides insight into the relationships between sequence, structure, dynamics and function among these EF-hand CaBPs.

Quercetin binds to calcineurin at a similar region to cyclosporin A and tacrolimus.

Quercetin, the primary dietary flavonol, exerts a strong inhibitory effect on calcineurin (CN), a unique Ca(2+)/calmodulin-dependent serine/threonine protein phosphatase. Using fluorescence spectroscopy (FS) we showed quercetin strongly bound to calcineurin catalytic subunit (CNA) with a ratio of 1:1; we also showed that calcineurin regulatory subunit (CNB) weakened this binding. In addition, the secondary structure of CNA was much tighter in the presence of quercetin. An FS study with CNA truncated mutant CNAa showed that the binding area for quercetin was reduced to the catalytic domain of CNA. Furthermore, fluorescence resonance energy transfer (FRET) results and molecular docking indicated three potential binding sites for quercetin, which were located at a region between the active centre of CNA and the CNB binding domain, a similar binding area to that of cyclosporin A and tacrolimus. Interestingly, this region was also important for CN substrate recognition.

Studies on the interactions of kaempferol to calcineurin by spectroscopic methods and docking.

Kaempferol, in our previous study, was a new immunosuppressant on calcineurin (CN), the Ca(2+)/calmodulin (CaM)-dependent protein phosphatase. Here, we examined the interactions of kaempferol with CN by fluorescence spectroscopy (FS), circular dichroism spectroscopy (CD) and docking. Data of kaempferol with CN catalytic subunit (CN A) and its truncated mutant CNAa obtained by FS method showed that the binding stoichiometry of kaempferol/CN A was 1:1, catalytic domain of CN A was the concrete domain for kaempferol binding while other domains contributed a lot to this binding. Distances from kaempferol to each tryptophan (Trp) in CN A by energy transfer experiments and the subsequent docking study interestingly provided the same binding sites for kaempferol, which all located in the non-active site area of CN A catalytic domain, also consisted with our previous conclusion from CN activity assay. Furthermore, CD results showed a much tighter structure of CN A for the inhibitor binding; on the other hand, presence of Ca(2+) and Mn(2+) decreased kaempferol binding on CN A.