Suramin and the suramin analogue NF307 discriminate among calmodulin-binding sites

Calmodulin-binding sites on target proteins show considerable variation in primary sequence; hence compounds that block the access of calmodulin to th ese binding sites may be more selective than compounds that inactivate, cal modulin. Suramin and its analogue NF307 inhibit the interaction of calmodul in with the ryanodine receptor. We have investigated whether inhibition of calmodulin binding to target proteins is a general property of these compou nds. Suramin inhibited binding of [I-125]calmodulin to porcine brain membra nes and to sarcoplasmic reticulum from skeletal muscle (IC50 = 4.9 +/- 1.2 muM and 19.9 +/- 1.8 muM, respectively) and blocked the cross-linking of [I -125]calmodulin to some, but not ail, target proteins in brain membranes by [I-125]calmodulin. Four calmodulin-binding proteins were purified [ryanodi ne receptor-1 (RyR1) from rabbit skeletal muscle, neuronal NO synthase (nNO S) from Sf9 cells, G-protein beta gamma dimers (G beta gamma) from porcine brain and a glutathione S-transferase-fusion protein comprising the C-termi nal calmodulin-binding domain of the metabotropic glutamate receptor 7A (GS T-CmGluR7A) from bacterial lysates]. Three of the proteins employed (G beta gamma, GST-CmGluR7A and RyR1) display a comparable affinity for calmodulin (in the range of 50-70 nM). Nevertheless, suramin and NF307 only blocked t he binding of G beta gamma and RyR1 to calmodulin-Sepharose. In contrast, t he association of GST-CmGluR7A and nNOS was not impaired, whereas excess ca lmodulin uniformly displaced all proteins from the matrix. Thus suramin and NF307 are prototypes of a new class of calmodulin antagonists that do not interact directly with calmodulin but with calmodulin-recognition sites. In addition, these compounds discriminate among calmodulin-binding motifs.