THERMODYNAMIC STUDY OF THE PH-DEPENDENT INTERACTION OF CHROMOGRANIN-A WITH AN INTRALUMINAL LOOP PEPTIDE OF THE INOSITOL 1,4,5-TRISPHOSPHATE RECEPTOR

The secretory vesicles of adrenal chromaffin cells have previously been identified as a major inositol 1,4,5-trisphosphate (IP3)-sensitive Ca2+ store, and their Ca2+ store role has been attributed to the presence of chromogranin A, a high capacity, low affinity Ca2+ binding protein. Chromogranin A has since been shown to exist primarily in a dimeric state at pH 7.5 and primarily in a tetrameric state at the intravesicular pH of 5.5 and has also been shown to interact with the membrane proteins of secretory vesicles at pH 5.5, including a 260-kDa protein reactive to IP3 receptor antibody [Yoo, S. H. (1994) J. Biol. Chem. 269, 12001-12006]. In a recent study, chromogranin A was shown to interact with one of the intraluminal loop regions of the IP3 receptor at pH 5.5 but not at pH 7.5 [Yoo, S. Il., and Lewis, M. S. (1994) FEES Lett. 341, 28-32]. To gain further insight, we have studied the temperature dependence of the pH-dependent interaction of chromogranin A with the intraluminal peptide of the the IP3 receptor by analytical ultracentrifugation, using multiwavelength scan analysis, and found that four molecules of the intraluminal domain peptide of the IP3 receptor bound to each chromogranin A tetramer with Delta G degrees values ranging from -23.6 to -27.6 kcal mol(-1) in the absence and presence of 35 mM Ca2+. In the presence of 35 mM Ca2+, the interaction between chromogranin A tetramer and the intraluminal domain peptide of the IP3 receptor showed Delta G degrees values of increasing magnitude as the temperature was increased while the same interaction in the absence of Ca2+ showed Delta G degrees values of decreasing magnitude. Further, the values for the Delta H degrees and Delta S degrees of the interaction increased in the absence of Ca2+ but decreased in the presence of 35 mM Ca2+ as the temperature was increased, indicating the stabilizing effect of Ca2+ on the interaction.