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Determinants for calmodulin binding on voltage-dependent Ca2+ channels
P Pate1, J Mochca-Morales, Y Wu
1Department of Molecular Physiology and Biophysics, Baylor College of Medicine, Houston, Texas 77030, USA.
Abstract:
Calmodulin, bound to the alpha(1) subunit of the cardiac L-type calcium channel, is required for calcium-dependent inactivation of this channel. Several laboratories have suggested that the site of interaction of calmodulin with the channel is an IQ-like motif in the carboxyl-terminal region of the alpha(1) subunit. Mutations in this IQ motif are linked to L-type Ca(2+) current (I(Ca)) facilitation and inactivation. IQ peptides from L, P/Q, N, and R channels all bind Ca(2+)calmodulin but not Ca(2+)-free calmodulin. Another peptide representing a carboxyl-terminal sequence found only in L-type channels (designated the CB domain) binds Ca(2+)calmodulin and enhances Ca(2+)-dependent I(Ca) facilitation in cardiac myocytes, suggesting the CB domain is functionally important. Calmodulin blocks the binding of an antibody specific for the CB sequence to the skeletal muscle L-type Ca(2+) channel, suggesting that this is a calmodulin binding site on the intact protein. The binding of the IQ and CB peptides to calmodulin appears to be competitive, signifying that the two sequences represent either independent or alternative binding sites for calmodulin rather than both sequences contributing to a single binding site.
Insights
Calmodulin binding to cardiac L-type calcium channels is crucial for calcium-dependent inactivation. The study identifies distinct IQ and CB domains as competitive calmodulin binding sites, impacting channel function.
Area of Science:
- Cardiovascular Physiology
- Molecular Biology
- Ion Channel Function
Background:
- Calmodulin (CaM) regulates cardiac L-type calcium channels (LTCCs), mediating calcium-dependent inactivation.
- The carboxyl-terminal IQ motif of LTCC alpha(1) subunits is proposed as a CaM interaction site.
- Mutations in the IQ motif affect L-type Ca(2+) current (I(Ca)) facilitation and inactivation.
Purpose of the Study:
- To investigate the functional significance of the CB domain in LTCCs.
- To determine the binding relationship between the IQ motif and the CB domain with CaM.
- To elucidate the molecular mechanisms of CaM-LTCC interaction.
Main Methods:
- Peptide synthesis and binding assays with Ca(2+)-bound and Ca(2+)-free calmodulin.
- Functional studies using cardiac myocytes to assess I(Ca) facilitation.
- Antibody binding assays to map CaM interaction sites on intact channels.
Main Results:
- Both IQ and CB peptides bind Ca(2+)-calmodulin, but not Ca(2+)-free calmodulin.
- The CB domain peptide enhances Ca(2+)-dependent I(Ca) facilitation in cardiac myocytes.
- Calmodulin binding to the CB sequence on intact channels was confirmed.
- Competitive binding assays indicate IQ and CB domains are distinct or alternative CaM binding sites.
Conclusions:
- The CB domain is functionally important for Ca(2+)-dependent I(Ca) facilitation in cardiac myocytes.
- IQ and CB domains represent independent or alternative binding sites for calmodulin on LTCCs.
- These findings refine our understanding of CaM-LTCC complex regulation.