Calcineurin B (CnB) and calmodulin (CaM) are two structurally similar but functionally distinct 'EF-hand' Ca2+-binding proteins. CnB is the regulatory subunit of the CaM-stimulated protein phosphatase, calcineurin. CaM is a unique multifunctional protein that interacts with and modulates the activity of many target proteins. CnB and CaM are both required for the full activation of the phosphatase activity of calcineurin and are not interchangeable. The two proteins recognize distinct binding sites on calcineurin A subunit (CnA) and perform different functions. Phage-displayed peptide libraries (pIII and pVIII libraries) were screened with CnB and CaM to isolate peptides that could then be compared to determine if there were binding preferences of the two proteins. The Ca2+-dependent binding of phage-displayed peptides to CnB and CaM is specifically blocked by synthetic peptides derived from the CnB-binding domain of CnA and the CaM-binding domain of myosin light chain kinase respectively. Both CnB- and CaM-binding peptides have a high content of tryptophan and leucine, but CnB-binding peptides are more hydrophobic than CaM-binding peptides. CnB-binding peptides are negatively charged with clusters of hydrophobic residues rich in phenylalanine, whereas the CaM-binding peptides are positively charged and often contain an Arg/Lys-Trp motif. The binding preferences identified with peptide libraries are consistent with the features of the CnB-binding domains of all CnA isoforms and the CaM-binding domains of CaM targets.
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