release through type 1 ryanodine receptors (RYR1) is a major mechanism underlying the earliest signals in T cell activation, which are the formation of Ca
microdomains. In our characterization of the molecular machinery underlying NAADP action, we identified an NAADP-binding protein, called hematological and neurological expressed 1-like protein (HN1L) [also known as Jupiter microtubule-associated homolog 2 (JPT2)]. Gene deletion of
in human Jurkat and primary rat T cells resulted in decreased numbers of initial Ca
microdomains and delayed the onset and decreased the amplitude of global Ca
signaling. Photoaffinity labeling demonstrated direct binding of NAADP to recombinant HN1L/JPT2. T cell receptor/CD3-dependent coprecipitation of HN1L/JPT2 with RYRs and colocalization of these proteins suggest that HN1L/JPT2 connects NAADP formation with the activation of RYR channels within the first seconds of T cell activation. Thus, HN1L/JPT2 enables NAADP to activate Ca
release from the endoplasmic reticulum through RYR.
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