Thermodynamic linkage between calmodulin domains binding calcium and contiguous sites in the C-terminal tail of Ca(V)1.2

• Published in: Biophysical chemistry 2011-11, Vol.159 (1), p.172-187
• Main Authors: Evans, T Idil Apak, Hell, Johannes W, Shea, Madeline A
• Format: Article
• Language: English
• Subjects: Amino Acid Sequence; Animals; Calcium - metabolism; Calcium Channels, L-Type - chemistry; Calcium Channels, L-Type - metabolism; Calmodulin - chemistry; Calmodulin - metabolism; Catalytic Domain; Models, Molecular; Molecular Sequence Data; Protein Binding; Rats; Thermodynamics
• ISSN: 1873-4200
• DOI: 10.1016/j.bpc.2011.06.007

Calmodulin (CaM) binding to the intracellular C-terminal tail (CTT) of the cardiac L-type Ca(2+) channel (Ca(V)1.2) regulates Ca(2+) entry by recognizing sites that contribute to negative feedback mechanisms for channel closing. CaM associates with Ca(V)1.2 under low resting [Ca(2+)], but is poised to change conformation and position when intracellular [Ca(2+)] rises. CaM binding Ca(2+), and the domains of CaM binding the CTT are linked thermodynamic functions. To better understand regulation, we determined the energetics of CaM domains binding to peptides representing pre-IQ sites A(1588), and C(1614) and the IQ motif studied as overlapping peptides IQ(1644) and IQ'(1650) as well as their effect on calcium binding. (Ca(2+))(4)-CaM bound to all four peptides very favorably (K(d)≤2 nM). Linkage analysis showed that IQ(1644-1670) bound with a K(d)~1 pM. In the pre-IQ region, (Ca(2+))(2)-N-domain bound preferentially to A(1588), while (Ca(2+))(2)-C-domain preferred C(1614). When bound to C(1614), calcium binding in the N-domain affected the tertiary conformation of the C-domain. Based on the thermodynamics, we propose a structural mechanism for calcium-dependent conformational change in which the linker between CTT sites A and C buckles to form an A-C hairpin that is bridged by calcium-saturated CaM.