The initial phosphorylation of JAK1 and JAK2 is mediated by JAK2. Autophosphorylated JAK2 may transphopshorylate JAK1 bound to IFNGR1 chain. Tyrosine 1033 in the activation loop of the JAK1 kinase domain may be the target for transphosphorylation (phosphorylation site mentioned here is based on sequence similarity with all the other JAK kinases).
Briscoe, J, Rogers, NC, Witthuhn, BA, Watling, D, Harpur, AG, Wilks, AF, Stark, GR, Ihle, JN, Kerr, IM
protein tyrosine kinase activity of IFNG:IFNGR1:JAK1:IFNGR2:p-JAK2 [plasma membrane]
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