The Tec family tyrosine kinase Itk is an important transducer of T cell activating signals. Itk regulates the levels of two second messengers Ca2+ and diacylglycerol, which are required for initiating transcriptional programs important for T cell priming and differentiation. How T cells translate intracellular signaling cascades into distinct functional outcomes remains a fundamentally important question. We hypothesize that distinct mechanisms of Itk activation lead to Ca2+ patterns that result in divergent T cell responses. Itk activation requires recruitment via its Pleckstrin homology (PH) domain to its membrane ligand PIP3, which is generated by PI-3 kinase. Here, we report a role for Calmodulin (CaM) in sustaining Itk activity and Ca2+ responses. CaM is a downstream Ca2+ sensor that controls numerous cellular enzymes, including Calcineurin. We have identified a novel interaction between the Itk PH domain and CaM. NMR analyses revealed that CaM binds adjacent to the PIP3 binding pocket, leading us to hypothesize that CaM regulates PIP3 binding. CaM inhibition or genetic mutation of the CaM binding site resulted in decreased PIP3 association and Itk activation, resulting in less sustained Ca2+ responses. We propose a model in which initial Itk activation results in low Ca2+ increases that feedback to boost membrane recruitment of Itk via CaM. We speculate that T cell responses requiring sustained Ca2+ signals, such as T cell differentiation require this feedback loop.
- Copyright © 2013 by The American Association of Immunologists, Inc.