IDENTIFICATION AND CHARACTERIZATION OF 2 DISTINCT CALMODULIN-BINDING SITES IN THE TRPL ION-CHANNEL PROTEIN OF DROSOPHILA-MELANOGASTER

Two putative light-sensitive ion channels have been isolated from Dros ophila, encoded by the transient-receptor-potential (trp) and transien t-receptor-potential-like (trpl) genes. The cDNA encoding the Trpl pro tein was initially isolated on the basis that the expressed protein bi nds calmodulin. Using both fusion proteins and a synthetic peptide, we now show that two calmodulin-binding sites are present in the C-termi nal domain of the Trpl protein, CBS-1 and CBS-2. CBS-1 binds calmoduli n in a Ca2+-dependent fashion, requiring Ca2+ concentrations above 0.3 -0.5 mu M for calmodulin binding. In contrast, CBS-2 binds the Ca2+-fr ee form of calmodulin, with dissociation occurring at Ca2+ concentrati ons between 5 and 25 mu M. Phosphorylation of a serine residue within a peptide encompassing CBS-1 by cyclic AMP-dependent protein kinase (P KA) abolishes calmodulin binding, and phosphorylation of the adjacent serine by protein kinase C appears to modulate this phosphorylation by PKA. Interpretation of these findings provides a novel model for ion- channel gating and modulation in response to changing levels of intrac ellular Ca2+.