The multi-site docking protein Gab1 is constitutively phosphorylated independent from its recruitment to the plasma membrane in Jak2-V617F-positive cells and mediates proliferation of human erythroleukaemia cells.

The constitutively active Janus kinase 2 mutant Jak2-V617F is responsible for cytokine-independent growth of hematopoietic cells and the development of myeloproliferative neoplasms, such as polycythaemia vera and essential thrombocythaemia. Cells expressing Jak2-V617F exhibit constitutive STAT, MAPK, and PI3K signalling, and constitutive association of the multi-site docking protein Gab1 to PIP3 at the plasma membrane. Here, we demonstrate the crucial role of Gab1 for the proliferation of Jak2-V617F-positive human erythroleukaemia (HEL) cells. In Jak2-V617F-expressing cells Gab1 is constitutively phosphorylated by Erk1/2 on serine residue 552, which regulates binding to PIP3. Additionally, Gab1 is constitutively phosphorylated on tyrosine residue 627. Tyrosine 627 is a SHP2 binding site and required for Gab1-dependent Erk1/2 activation. As previously shown, Jak2-V617F-dependent Erk1/2 and PI3K activation act synergistically on the proliferation of Jak2-V617F-positive cells. Here, we examined whether constitutive membrane association of Gab1 explains cytokine-independent Gab1 phosphorylation in Jak2-V617F-expressing cells. Although we could demonstrate Jak2-V617F-dependent constitutive serine 552 and tyrosine 627 phosphorylation of Gab1, interestingly, both phosphorylations do not require binding of Gab1 to PIP3 at the plasma membrane. Instead, we observed a constitutive interaction of Gab1 with the erythropoietin receptor in Jak2-V617F-expressing cells, which depends on Janus kinase activity. Thus, constitutive Gab1-dependent signalling in Jak2-V617F-expressing cells does not occur due to the constitutive association of Gab1 with PIP3 at the plasma membrane.