Calcium (Ca) serves as a critical signaling molecule involved in numerous regulatory pathways including T cell activation, smooth muscle contraction, hormone release, cell growth and differentiation and fertilization. The inositol 1 ,4,5-trisphosphate receptor (lP3R) is the major intracellular Ca release channel in non-excitable cells, In many cases stimulation of a plasma membrane receptor induces generation of lP3 that activates the lP3R to release Ca from intracellular stores. Despite the central importance of the lP3-gated Ca channel, much remains to be learned about how the channel is regulated. We have cloned the complete cDNA encoding the human type 1 lP3R (lP3R1) from T lymphocytes (Jurkat). We have identified putative tyrosine phosphorylation sites in the deduced amino acid sequence of IP3R1 and shown that the channel is tyrosine phosphorylated and becomes physically associated with fyn in activated T cells. Moreover, we have shown that tyrosine phosphorylation by fyn increases the open probability of lP3R in planar lipid bilayers. More recently, we have been able to express functional human recombinant IP3R1 in heterologous systems that are suitable for characterization of channel properties. These observations have lead us to ask the following questions: 1) which residues are tyrosine phosphorylated in lP3R1; 2) does lP3R1 interact with the SH2 binding domain of members of the src family kinases; 3) does tyrosine phosphorylation of IP3R1 alter the Ca- sensitivity for activation and inactivation of the channel; and 4) what are the relative roles of src family kinases (including fyn and lck) in regulating lP3R1 in T lymphocytes? These studies should lead to novel information regarding the molecular mechanisms by which tyrosine phosphorylation regulates IP3R1 function. Understanding IP3R regulation should provide insights into signaling pathways that are potential therapeutic targets for immunosuppression, cancer, birth control, hypertension, and stroke.
