THE NEURONAL CALCIUM-SENSOR PROTEIN VILIP MODULATES CYCLIC-AMP ACCUMULATION IN STABLY TRANSFECTED C6 GLIOMA-CELLS - AMINO-TERMINAL MYRISTOYLATION DETERMINES FUNCTIONAL-ACTIVITY

VILIP (visinin-like protein) is a member of the neuronal subfamily of EF-hand calcium sensor proteins. Members of this family are involved i n the calcium-dependent regulation of the desensitization of signal ca scades in retinal photoreceptors. To gain insight into the function of VILIP in cell signaling, we have transfected wild-type VILIP and muta nt VILIP lacking the myristoylation consensus sequence into C6 glioma cells. Expression of wildtype VILIP did not significantly influence th e desensitization of beta-adrenergic receptors, which are coupled to a denylyl cyclase in C6 cells. However, VILIP expression increased the b eta-adrenergic receptor-stimulated cyclic AMP (cAMP) level in these ce lls severalfold. The stimulatory effect was also observed after direct stimulation of the adenylyl cyclase with forskolin, indicating that V ILIP acts downstream of receptor and G protein in the beta-adrenergic signaling pathway in C6 cells. In contrast, the nonmyristoylated mutan t of VILIP reduced cellular cAMP levels in C6 cells. Myristoylated wil d-type VILIP was associated in a calcium-dependent manner with membran e fractions during subcellular fractionation, presumably owing to a ca lcium-myristoyl switch, In contrast, association of nonmyristoylated m utant VILIP with membranes was strongly reduced. Thus, myristoylation and most likely the calcium-dependent membrane association of VILIP ar e important prerequisites for the activating effect of wild-type VILIP on cAMP accumulation in C6 cells. These results suggest that VILIP ac ts as a calcium sensor molecule that modulates cell signaling cascades , possibly by direct or indirect regulation of adenylyl cyclase activi ty.