There is communication between all four Ca(2+)-bindings sites of calcineurin B
1Bioscience Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA.
Abstract:
We have used site-directed mutagenesis, flow dialysis, and Fourier transform infrared (FTIR) spectroscopy to study Ca(2+)-binding to the regulatory component of calcineurin. Single Glu-Gln(E --> Q) mutations were used to inactivate each of the four Ca(2+)-binding sites of CnB in turn, generating mutants Q1, Q2, Q3, and Q4, with the number indicating which Ca(2+) site is inactivated. The binding data derived from flow dialysis reveal two pairs of sites in the wild-type protein, one pair with very high affinity and the other with lower affinity Ca(2+)-binding sites. Also, only three sites are titratable in the wild-type protein because one site cannot be decalcified. Mutation of site 2 leaves the protein with only two titratable sites, while mutation of sites 1, 3, or 4 leave three titratable sites that are mostly filled with 3 Ca(2+) equiv added. The binding data further show that each of the single-site mutations Q2, Q3, and Q4 affects the affinities of at least one of the remaining sites. Mutation in either of sites 3 or 4 results in a protein with no high-affinity sites, indicating communication between the two high-affinity sites, most likely sites 3 and 4. Mutation in site 2 decreases the affinity of all three remaining sites, though still leaving two relatively high-affinity sites. The FTIR data support the conclusions from the binding data with respect to the number of titratable sites as well as the impact of each mutation on the affinities of the remaining sites. We conclude therefore that there is communication between all four Ca(2+)-binding sites. In addition, the Ca(2+) induced changes in the FTIR spectra for the wild-type and Q4 mutant are most similar, suggesting that the same three Ca(2+)-binding sites are being titrated, i.e., site 4 is the very high-affinity site under the conditions of the FTIR experiments.
Insights
Site-directed mutagenesis revealed communication between all four calcium-binding sites in calcineurin B (CnB). Mutations affected Ca(2+) binding affinities, indicating interconnectedness of these regulatory sites.
Area of Science:
- Biochemistry
- Molecular Biology
- Protein Structure and Function
Background:
- Calcineurin B (CnB) is a regulatory component of calcineurin, a crucial calcium-dependent phosphatase.
- CnB contains four calcium-binding sites that are essential for its function.
- Understanding the interplay between these calcium-binding sites is key to elucidating calcineurin regulation.
Purpose of the Study:
- To investigate the functional communication between the four Ca(2+)-binding sites in calcineurin B (CnB).
- To determine the impact of individual Ca(2+)-binding site mutations on Ca(2+) affinity and titratability.
- To correlate spectroscopic and binding data for a comprehensive understanding of CnB's Ca(2+) regulation.
Main Methods:
- Site-directed mutagenesis was employed to generate single Glu-Gln mutations (Q1-Q4) in each Ca(2+)-binding site of CnB.
- Flow dialysis was used to quantify Ca(2+) binding affinities and determine the number of titratable sites in wild-type and mutant CnB.
- Fourier transform infrared (FTIR) spectroscopy was utilized to analyze Ca(2+)-induced conformational changes and corroborate binding data.
Main Results:
- Wild-type CnB exhibits two pairs of Ca(2+)-binding sites: one high-affinity pair and one lower-affinity pair. Only three sites are titratable.
- Mutations Q2, Q3, and Q4 each affected the affinities of remaining Ca(2+)-binding sites, demonstrating inter-site communication.
- Mutations in sites 3 or 4 abolished high-affinity binding, suggesting communication between these two sites. Mutation in site 2 decreased affinities of all remaining sites.
- FTIR data supported binding results, indicating communication between all four Ca(2+)-binding sites.
Conclusions:
- There is significant communication between all four Ca(2+)-binding sites in calcineurin B.
- The affinities and titratability of Ca(2+)-binding sites are interdependent.
- Site 4 appears to be the very high-affinity site under FTIR experimental conditions, with titration of sites 1, 2, and 3.
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