We have previously demonstrated that calmodulin (CaM) binds directly to c-FLIP L in a Ca 2+-dependent manner. Deletion of the CaM-binding region (amino acid 197-213) results in reduced CaM binding, and increased Fas-mediated apoptosis and decreased tumorigenesis of cholangiocarcinoma cells. The present studies were designed to identify the precise amino acids between 197 and 213 that are responsible for CaM/FLIP binding, and their roles in mediating the anti-apoptotic function of c-FLIP L. Sequence analysis of the CaM-binding region at 197-213 predicted three unique positively charged residues at 204, 207 and 209, which might be responsible for the CaM/FLIP binding. A point mutation at H204 of c-FLIP L was found to markedly reduce CaM binding, whereas point mutation at R207 or K209 did not affect c-FLIP L binding to CaM. Decreased CaM/FLIP binding was confirmed in cholangiocarcinoma cells overexpressing the H204 c-FLIP L mutant. Reduced CaM binding by the H204 mutant resulted in increased sensitivity to Fas-mediated apoptosis and inhibited tumor growth in mice compared with wild-type c-FLIP L. Death-inducing signaling complex (DISC) analysis showed that the reduced CaM binding to H204 mutant resulted in less c-FLIP L recruited into the DISC. Concurrently, increased caspase 8 was recruited to the DISC, which resulted in increased cleavage and activation of caspase 8, activation of downstream caspase 3 and increased apoptosis. Therefore, these results demonstrate that the H204 residue is responsible for c-FLIP L binding to CaM, which mediates the anti-apoptotic function of c-FLIP L, most likely through affecting recruitment of caspase 8 into the DISC and thus caspase 8 activation. These studies further characterized CaM/FLIP interaction and its function in regulating Fas-mediated apoptosis and tumorigenesis, which may provide new therapeutic targets for cancer therapy. © 2012 USCAP, Inc All rights reserved.