Autosomal SCID caused by a point mutation in the N-terminus of Jak3: mapping of the Jak3-receptor interaction domain

Signaling through the hematopoietic receptors requires activation of recept or-associated Janus (Jak) kinases, For example, Jak1 and Jak3 bind specific ally to the IL-2 receptor beta (IL-2R beta) and common gamma (gamma(c)) cha ins, respectively, and initiate biochemical signals critical in controlling immune responses. The region of Jak responsible for receptor interactions, however, is not well characterized. Here we describe a naturally occurring Jak3 mutation from a patient with autosomal severe combined immunodeficien cy (SCID), where a single amino acid substitution, Y100C, in Janus homology domain 7 (JH7) prevents kinase-receptor interaction. This mutation also re sults in a loss of IL-2-induced signaling in a B-cell line derived from thi s patient. Using mutational analysis we have identified a region of Jak3, i ncluding portions of JH6 and JH7, that is sufficient for kinase-receptor co ntact and show that this segment interacts with the proline-rich Box1 regio n of the receptor. Furthermore, a Jak3-Jak1 chimera containing only the JH6 and JH7 domains of Jak3 interacts with gamma(c) and can reconstitute IL-2- dependent responses, including receptor phosphorylation and activation of s ignal transducer and activator of transcription (STAT) 5b, Our results sugg est that the N-terminus of Jak kinases is critical for receptor binding, an d is therefore likely to determine specificity of Jak kinase-receptor inter actions.