Tyrosine 813 is required for SH2-Bβ to enhance phosphorylation of JAK2. COS7 cells were transfected with 1.0 μg of cDNA encoding JAK2 (lanes 1 and 2), JAK2(K882E) (lanes 3 and 4), JAK2(Y813F) (lanes 5 and 6), or JAK2(Y570F) (lanes 7 and 8), with (lanes 2, 4, 6, and 8) or without (lanes 1, 3, 5, and 7) 2.0 μg of cDNA encoding myc-SH2-Bβ. In the absence of myc-SH2-Bβ, cells were transfected with 2.0 μg of empty prk5 myc vector. JAK2 was immunoprecipitated (IP) with anti-JAK2 and immunoblotted with anti-PY (upper panel) and anti-JAK2 (middle panel). For the lower panel, cell lysates of the above transfected cells were immunoblotted with anti-myc to assess levels of expression of myc-SH2-Bβ.