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C Terminus L‐type Ca 2+ Channel Calmodulin‐Binding Domains are ‘Auto‐Agonist’ Ligands in Rabbit Ventricular Myocytes
Author(s) -
Dzhura Igor,
Wu Yuejin,
Zhang Rong,
Colbran Roger J.,
Hamilton Susan L.,
Anderson Mark E.
Publication year - 2003
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1113/jphysiol.2003.043778
Subject(s) - agonist , calmodulin , chemistry , myocyte , rabbit (cipher) , receptor , biophysics , endocrinology , microbiology and biotechnology , medicine , biology , biochemistry , computer science , enzyme , computer security
L‐type Ca 2+ channel C terminus calmodulin (CaM)‐binding domains are molecular determinants for Ca 2+ –CaM‐dependent increases in L‐type Ca 2+ current ( I Ca ), and a CaM‐binding IQ domain mimetic peptide (IQmp) increases L‐type Ca 2+ channel current by promoting a gating mode with prolonged openings (mode 2), suggesting the intriguing possibility that CaM‐binding domains are ‘auto‐agonist’ signalling molecules. In order to test the breadth of this concept, we studied the effect of a second C terminus CaM‐binding domain (CB) mp (CBmp), in conjunction with IQmp, on single L‐type Ca 2+ channel currents in excised cell membrane patches from rabbit ventricular myocytes. Here we show that both CBmp and IQmp are agonist ligands that non‐additively increase L‐type Ca 2+ channel opening probability ( P o ) by inducing mode 2 gating. CBmp and IQmp agonist effects were lost under conditions favouring calcification of CaM (Ca 2+ –CaM, 150 n m free Ca 2+ and 10–20 μ m CaM), but persisted in the presence of CaM (0–20 μ m ) under conditions adverse to Ca 2+ –CaM (20 m m BAPTA), indicating that CaM‐binding domains increase L‐type Ca 2+ channel P o by a low Ca 2+ –CaM activity mechanism. Increasing Ca 2+ –CaM in the bath (cytosol) reduced the efficacy of CBmp and IQmp signals with Ba 2+ as charge carrier, suggesting that CaM binding motifs target a site outside of the pore region. We measured the combined effects of CBmp and Ca 2+ –CaM‐dependent protein kinase II (CaMKII) on L‐type Ca 2+ channels by using an engineered Ca 2+ –CaM‐independent form of CaMKII that remains active under low Ca 2+ –CaM conditions, permissive for CBmp signalling. CBmp and CaMKII increased L‐type Ca 2+ channel P o in a non‐additive manner, suggesting that low and high Ca 2+ –CaM‐dependent L‐type Ca 2+ channel facilitation pathways converge upon a common signalling mechanism.