Phosphorylated residues in the cytoplasmic region of Interleukin-12 receptor subunit beta 2 (IL12RB2) enable binding and activation of Signal transducer and activator of transcription 4 (STAT4) by means of Tyrosine-protein kinase JAK2 (JAK2).
STAT4 was found to bind only phosphorylated tyrosine-800, and not to phosphorylated tyrosines 678 or 767 when phosphorylated. Although required, phosphorylated tyrosine-800 was not sufficient for binding of STAT4, as other residues in IL12RB2, specifically at -4, -1 and +1 relative to the tyrosine, contribute to the specificity of STAT4 binding (Yao et al. 1999).
As the pattern of phosphorylations required for receptor activation is unknown, this is a black box event.