IQGAP1 integrates Ca2+/calmodulin and B-Raf signaling.
Ca(2+) and calmodulin modulate numerous cellular functions, ranging from muscle contraction to the cell cycle. Accumulating evidence indicates that Ca(2+) and calmodulin regulate the MAPK signaling pathway at multiple positions in the cascade, but the molecular mechanism underlying these observations is poorly defined. We previously documented that IQGAP1 is a scaffold in the MAPK cascade. IQGAP1 binds to and regulates the activities of ERK, MEK, and B-Raf. Here we demonstrate that IQGAP1 integrates Ca(2+) and calmodulin with B-Raf signaling. In vitro analysis reveals that Ca(2+) promotes the direct binding of IQGAP1 to B-Raf. This interaction is inhibited by calmodulin in a Ca(2+)-regulated manner. Epidermal growth factor (EGF) is unable to stimulate B-Raf activity in fibroblasts treated with the Ca(2+) ionophore A23187. In contrast, chelation of intracellular free Ca(2+) concentrations ([Ca(2+)](i)) significantly enhances EGF-stimulated B-Raf activity, an effect that is dependent on IQGAP1. Incubation of cells with EGF augments the association of B-Raf with IQGAP1. Moreover, Ca(2+) regulates the association of B-Raf with IQGAP1 in cells. Increasing [Ca(2+)](i) with Ca(2+) ionophores significantly reduces co-immunoprecipitation of B-Raf and IQGAP1, whereas chelation of Ca(2+) enhances the interaction. Consistent with these findings, increasing and decreasing [Ca(2+)](i) increase and decrease, respectively, co-immunoprecipitation of calmodulin with IQGAP1. Collectively, our data identify a previously unrecognized mechanism in which the scaffold protein IQGAP1 couples Ca(2+) and calmodulin signaling to B-Raf function.
Ren
Jian-Guo
JG
Department of Pathology, Brigham and Women's Hospital, Boston, Massachusetts 02115, USA.
Li
Zhigang
Z
Sacks
David B
DB
D000818:Animals; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000737:chemistry* / Q000378:metabolism; D002147:Calmodulin / Q000378:metabolism*; D004815:Epidermal Growth Factor / Q000378:metabolism; D005347:Fibroblasts / Q000378:metabolism; D005786:Gene Expression Regulation; D006801:Humans; D007476:Ionophores / Q000494:pharmacology; D051379:Mice; D008954:Models, Biological; D048493:Proto-Oncogene Proteins B-raf / Q000378:metabolism*; D015398:Signal Transduction; D020703:ras GTPase-Activating Proteins / Q000378:metabolism / Q000502:physiology*
D002147:Calmodulin; C101203:IQ motif containing GTPase activating protein 1; D007476:Ionophores; D020703:ras GTPase-Activating Proteins; D000001:Calcimycin; D004815:Epidermal Growth Factor; C482119:BRAF protein, human; C482120:Braf protein, mouse; D048493:Proto-Oncogene Proteins B-raf; D002118:Calcium